diff --git a/cli/tests/cli_run.rs b/cli/tests/cli_run.rs
index 5b8ec1acce..63996d3c92 100644
--- a/cli/tests/cli_run.rs
+++ b/cli/tests/cli_run.rs
@@ -227,6 +227,22 @@ mod cli_run {
         );
     }
 
+
+    #[test]
+    #[ignore]
+    #[serial(astar)]
+    fn run_astar_optimized_1() {
+        check_output_with_stdin(
+            &example_file("benchmarks", "AStarTests.roc"),
+            "1",
+            "astar-tests",
+            &[],
+            "True\n",
+            false,
+        );
+    }
+
+    #[ignore]
     #[test]
     #[serial(closure1)]
     fn closure1() {
diff --git a/compiler/build/src/program.rs b/compiler/build/src/program.rs
index e97c015d09..ee0d990ce5 100644
--- a/compiler/build/src/program.rs
+++ b/compiler/build/src/program.rs
@@ -97,6 +97,10 @@ pub fn gen_from_mono_module(
         if name.starts_with("roc_builtins.dict") || name.starts_with("dict.RocDict") {
             function.add_attribute(AttributeLoc::Function, attr);
         }
+
+        if name.starts_with("roc_builtins.list") || name.starts_with("list.RocList") {
+            function.add_attribute(AttributeLoc::Function, attr);
+        }
     }
 
     let builder = context.create_builder();
diff --git a/compiler/builtins/bitcode/src/dict.zig b/compiler/builtins/bitcode/src/dict.zig
index 4580e04176..f6ae1913db 100644
--- a/compiler/builtins/bitcode/src/dict.zig
+++ b/compiler/builtins/bitcode/src/dict.zig
@@ -5,9 +5,10 @@ const mem = std.mem;
 const Allocator = mem.Allocator;
 const assert = std.debug.assert;
 
+const utils = @import("utils.zig");
+const RocList = @import("list.zig").RocList;
+
 const INITIAL_SEED = 0xc70f6907;
-const REFCOUNT_ONE_ISIZE: comptime isize = std.math.minInt(isize);
-const REFCOUNT_ONE: usize = @bitCast(usize, REFCOUNT_ONE_ISIZE);
 
 const InPlace = packed enum(u8) {
     InPlace,
@@ -92,6 +93,23 @@ const Alignment = packed enum(u8) {
     }
 };
 
+pub fn decref(
+    allocator: *Allocator,
+    alignment: Alignment,
+    bytes_or_null: ?[*]u8,
+    data_bytes: usize,
+) void {
+    return utils.decref(allocator, alignment.toUsize(), bytes_or_null, data_bytes);
+}
+
+pub fn allocateWithRefcount(
+    allocator: *Allocator,
+    alignment: Alignment,
+    data_bytes: usize,
+) [*]u8 {
+    return utils.allocateWithRefcount(allocator, alignment.toUsize(), data_bytes);
+}
+
 pub const RocDict = extern struct {
     dict_bytes: ?[*]u8,
     dict_entries_len: usize,
@@ -211,7 +229,7 @@ pub const RocDict = extern struct {
 
         // otherwise, check if the refcount is one
         const ptr: [*]usize = @ptrCast([*]usize, @alignCast(8, self.dict_bytes));
-        return (ptr - 1)[0] == REFCOUNT_ONE;
+        return (ptr - 1)[0] == utils.REFCOUNT_ONE;
     }
 
     pub fn capacity(self: RocDict) usize {
@@ -228,8 +246,6 @@ pub const RocDict = extern struct {
         }
 
         // unfortunately, we have to clone
-
-        const in_place = InPlace.Clone;
         var new_dict = RocDict.allocate(allocator, self.number_of_levels, self.dict_entries_len, alignment, key_width, value_width);
 
         var old_bytes: [*]u8 = @ptrCast([*]u8, self.dict_bytes);
@@ -260,6 +276,10 @@ pub const RocDict = extern struct {
     }
 
     fn setKey(self: *RocDict, index: usize, alignment: Alignment, key_width: usize, value_width: usize, data: Opaque) void {
+        if (key_width == 0) {
+            return;
+        }
+
         const offset = blk: {
             if (alignment.keyFirst()) {
                 break :blk (index * key_width);
@@ -289,6 +309,10 @@ pub const RocDict = extern struct {
     }
 
     fn setValue(self: *RocDict, index: usize, alignment: Alignment, key_width: usize, value_width: usize, data: Opaque) void {
+        if (value_width == 0) {
+            return;
+        }
+
         const offset = blk: {
             if (alignment.keyFirst()) {
                 break :blk (self.capacity() * key_width) + (index * value_width);
@@ -518,11 +542,6 @@ pub fn elementsRc(dict: RocDict, alignment: Alignment, key_width: usize, value_w
     }
 }
 
-pub const RocList = extern struct {
-    bytes: ?[*]u8,
-    length: usize,
-};
-
 pub fn dictKeys(dict: RocDict, alignment: Alignment, key_width: usize, value_width: usize, inc_key: Inc, output: *RocList) callconv(.C) void {
     const size = dict.capacity();
 
@@ -538,7 +557,7 @@ pub fn dictKeys(dict: RocDict, alignment: Alignment, key_width: usize, value_wid
     }
 
     if (length == 0) {
-        output.* = RocList{ .bytes = null, .length = 0 };
+        output.* = RocList.empty();
         return;
     }
 
@@ -587,7 +606,7 @@ pub fn dictValues(dict: RocDict, alignment: Alignment, key_width: usize, value_w
     }
 
     if (length == 0) {
-        output.* = RocList{ .bytes = null, .length = 0 };
+        output.* = RocList.empty();
         return;
     }
 
@@ -621,82 +640,131 @@ pub fn dictValues(dict: RocDict, alignment: Alignment, key_width: usize, value_w
     output.* = RocList{ .bytes = ptr, .length = length };
 }
 
-fn decref(
-    allocator: *Allocator,
-    alignment: Alignment,
-    bytes_or_null: ?[*]u8,
-    data_bytes: usize,
-) void {
-    var bytes = bytes_or_null orelse return;
+fn doNothing(ptr: Opaque) callconv(.C) void {
+    return;
+}
 
-    const usizes: [*]usize = @ptrCast([*]usize, @alignCast(8, bytes));
+pub fn dictUnion(dict1: RocDict, dict2: RocDict, alignment: Alignment, key_width: usize, value_width: usize, hash_fn: HashFn, is_eq: EqFn, inc_key: Inc, inc_value: Inc, output: *RocDict) callconv(.C) void {
+    output.* = dict1.makeUnique(std.heap.c_allocator, alignment, key_width, value_width);
 
-    const refcount = (usizes - 1)[0];
-    const refcount_isize = @bitCast(isize, refcount);
+    var i: usize = 0;
+    while (i < dict2.capacity()) : (i += 1) {
+        switch (dict2.getSlot(i, key_width, value_width)) {
+            Slot.Filled => {
+                const key = dict2.getKey(i, alignment, key_width, value_width);
 
-    switch (alignment.toUsize()) {
-        8 => {
-            if (refcount == REFCOUNT_ONE) {
-                allocator.free((bytes - 8)[0 .. 8 + data_bytes]);
-            } else if (refcount_isize < 0) {
-                (usizes - 1)[0] = refcount + 1;
-            }
-        },
-        16 => {
-            if (refcount == REFCOUNT_ONE) {
-                allocator.free((bytes - 16)[0 .. 16 + data_bytes]);
-            } else if (refcount_isize < 0) {
-                (usizes - 1)[0] = refcount + 1;
-            }
-        },
-        else => unreachable,
+                switch (output.findIndex(alignment, key, key_width, value_width, hash_fn, is_eq)) {
+                    MaybeIndex.not_found => {
+                        const value = dict2.getValue(i, alignment, key_width, value_width);
+                        inc_value(value);
+
+                        // we need an extra RC token for the key
+                        inc_key(key);
+
+                        const dec_key = doNothing;
+                        const dec_value = doNothing;
+
+                        dictInsert(output.*, alignment, key, key_width, value, value_width, hash_fn, is_eq, dec_key, dec_value, output);
+                    },
+                    MaybeIndex.index => |_| {
+                        // the key is already in the output dict
+                        continue;
+                    },
+                }
+            },
+            else => {},
+        }
     }
 }
 
-fn allocateWithRefcount(
-    allocator: *Allocator,
-    alignment: Alignment,
-    data_bytes: usize,
-) [*]u8 {
-    comptime const result_in_place = InPlace.Clone;
+pub fn dictIntersection(dict1: RocDict, dict2: RocDict, alignment: Alignment, key_width: usize, value_width: usize, hash_fn: HashFn, is_eq: EqFn, dec_key: Inc, dec_value: Inc, output: *RocDict) callconv(.C) void {
+    output.* = dict1.makeUnique(std.heap.c_allocator, alignment, key_width, value_width);
 
-    switch (alignment.toUsize()) {
-        8 => {
-            const length = @sizeOf(usize) + data_bytes;
+    var i: usize = 0;
+    const size = dict1.capacity();
+    while (i < size) : (i += 1) {
+        switch (output.getSlot(i, key_width, value_width)) {
+            Slot.Filled => {
+                const key = dict1.getKey(i, alignment, key_width, value_width);
 
-            var new_bytes: []align(8) u8 = allocator.alignedAlloc(u8, 8, length) catch unreachable;
-
-            var as_usize_array = @ptrCast([*]usize, new_bytes);
-            if (result_in_place == InPlace.InPlace) {
-                as_usize_array[0] = @intCast(usize, number_of_slots);
-            } else {
-                as_usize_array[0] = REFCOUNT_ONE;
-            }
-
-            var as_u8_array = @ptrCast([*]u8, new_bytes);
-            const first_slot = as_u8_array + @sizeOf(usize);
-
-            return first_slot;
-        },
-        16 => {
-            const length = 2 * @sizeOf(usize) + data_bytes;
-
-            var new_bytes: []align(16) u8 = allocator.alignedAlloc(u8, 16, length) catch unreachable;
-
-            var as_usize_array = @ptrCast([*]usize, new_bytes);
-            if (result_in_place == InPlace.InPlace) {
-                as_usize_array[0] = 0;
-                as_usize_array[1] = @intCast(usize, number_of_slots);
-            } else {
-                as_usize_array[0] = 0;
-                as_usize_array[1] = REFCOUNT_ONE;
-            }
-
-            var as_u8_array = @ptrCast([*]u8, new_bytes);
-            const first_slot = as_u8_array + 2 * @sizeOf(usize);
-
-            return first_slot;
-        },
-        else => unreachable,
+                switch (dict2.findIndex(alignment, key, key_width, value_width, hash_fn, is_eq)) {
+                    MaybeIndex.not_found => {
+                        dictRemove(output.*, alignment, key, key_width, value_width, hash_fn, is_eq, dec_key, dec_value, output);
+                    },
+                    MaybeIndex.index => |_| {
+                        // keep this key/value
+                        continue;
+                    },
+                }
+            },
+            else => {},
+        }
     }
 }
+
+pub fn dictDifference(dict1: RocDict, dict2: RocDict, alignment: Alignment, key_width: usize, value_width: usize, hash_fn: HashFn, is_eq: EqFn, dec_key: Inc, dec_value: Inc, output: *RocDict) callconv(.C) void {
+    output.* = dict1.makeUnique(std.heap.c_allocator, alignment, key_width, value_width);
+
+    var i: usize = 0;
+    const size = dict1.capacity();
+    while (i < size) : (i += 1) {
+        switch (output.getSlot(i, key_width, value_width)) {
+            Slot.Filled => {
+                const key = dict1.getKey(i, alignment, key_width, value_width);
+
+                switch (dict2.findIndex(alignment, key, key_width, value_width, hash_fn, is_eq)) {
+                    MaybeIndex.not_found => {
+                        // keep this key/value
+                        continue;
+                    },
+                    MaybeIndex.index => |_| {
+                        dictRemove(output.*, alignment, key, key_width, value_width, hash_fn, is_eq, dec_key, dec_value, output);
+                    },
+                }
+            },
+            else => {},
+        }
+    }
+}
+
+pub fn setFromList(list: RocList, alignment: Alignment, key_width: usize, value_width: usize, hash_fn: HashFn, is_eq: EqFn, dec_key: Dec, output: *RocDict) callconv(.C) void {
+    output.* = RocDict.empty();
+
+    var ptr = @ptrCast([*]u8, list.bytes);
+
+    const dec_value = doNothing;
+    const value = null;
+
+    const size = list.length;
+    var i: usize = 0;
+    while (i < size) : (i += 1) {
+        const key = ptr + i * key_width;
+        dictInsert(output.*, alignment, key, key_width, value, value_width, hash_fn, is_eq, dec_key, dec_value, output);
+    }
+
+    // NOTE: decref checks for the empty case
+    const data_bytes = size * key_width;
+    decref(std.heap.c_allocator, alignment, list.bytes, data_bytes);
+}
+
+const StepperCaller = fn (?[*]u8, ?[*]u8, ?[*]u8, ?[*]u8, ?[*]u8) callconv(.C) void;
+pub fn dictWalk(dict: RocDict, stepper: Opaque, stepper_caller: StepperCaller, accum: Opaque, alignment: Alignment, key_width: usize, value_width: usize, accum_width: usize, output: Opaque) callconv(.C) void {
+    @memcpy(output orelse unreachable, accum orelse unreachable, accum_width);
+
+    var i: usize = 0;
+    const size = dict.capacity();
+    while (i < size) : (i += 1) {
+        switch (dict.getSlot(i, key_width, value_width)) {
+            Slot.Filled => {
+                const key = dict.getKey(i, alignment, key_width, value_width);
+                const value = dict.getValue(i, alignment, key_width, value_width);
+
+                stepper_caller(stepper, key, value, output, output);
+            },
+            else => {},
+        }
+    }
+
+    const data_bytes = dict.capacity() * slotSize(key_width, value_width);
+    decref(std.heap.c_allocator, alignment, dict.dict_bytes, data_bytes);
+}
diff --git a/compiler/builtins/bitcode/src/list.zig b/compiler/builtins/bitcode/src/list.zig
new file mode 100644
index 0000000000..8ea824fa31
--- /dev/null
+++ b/compiler/builtins/bitcode/src/list.zig
@@ -0,0 +1,280 @@
+const std = @import("std");
+const utils = @import("utils.zig");
+const RocResult = utils.RocResult;
+const mem = std.mem;
+const Allocator = mem.Allocator;
+
+const EqFn = fn (?[*]u8, ?[*]u8) callconv(.C) bool;
+const Opaque = ?[*]u8;
+
+pub const RocList = extern struct {
+    bytes: ?[*]u8,
+    length: usize,
+
+    pub fn len(self: RocList) usize {
+        return self.length;
+    }
+
+    pub fn isEmpty(self: RocList) bool {
+        return self.len() == 0;
+    }
+
+    pub fn empty() RocList {
+        return RocList{ .bytes = null, .length = 0 };
+    }
+
+    pub fn isUnique(self: RocList) bool {
+        // the empty list is unique (in the sense that copying it will not leak memory)
+        if (self.isEmpty()) {
+            return true;
+        }
+
+        // otherwise, check if the refcount is one
+        const ptr: [*]usize = @ptrCast([*]usize, @alignCast(8, self.bytes));
+        return (ptr - 1)[0] == utils.REFCOUNT_ONE;
+    }
+
+    pub fn allocate(
+        allocator: *Allocator,
+        alignment: usize,
+        length: usize,
+        element_size: usize,
+    ) RocList {
+        const data_bytes = length * element_size;
+
+        return RocList{
+            .bytes = utils.allocateWithRefcount(allocator, alignment, data_bytes),
+            .length = length,
+        };
+    }
+
+    pub fn makeUnique(self: RocList, allocator: *Allocator, alignment: usize, element_width: usize) RocList {
+        if (self.isEmpty()) {
+            return self;
+        }
+
+        if (self.isUnique()) {
+            return self;
+        }
+
+        // unfortunately, we have to clone
+        var new_list = RocList.allocate(allocator, self.length, alignment, element_width);
+
+        var old_bytes: [*]u8 = @ptrCast([*]u8, self.bytes);
+        var new_bytes: [*]u8 = @ptrCast([*]u8, new_list.bytes);
+
+        const number_of_bytes = self.len() * element_width;
+        @memcpy(new_bytes, old_bytes, number_of_bytes);
+
+        // NOTE we fuse an increment of all keys/values with a decrement of the input dict
+        const data_bytes = self.len() * element_width;
+        utils.decref(allocator, alignment, self.bytes, data_bytes);
+
+        return new_list;
+    }
+
+    pub fn reallocate(
+        self: RocList,
+        allocator: *Allocator,
+        alignment: usize,
+        new_length: usize,
+        element_width: usize,
+    ) RocList {
+        const old_length = self.length;
+        const delta_length = new_length - old_length;
+
+        const data_bytes = new_capacity * slot_size;
+        const first_slot = allocateWithRefcount(allocator, alignment, data_bytes);
+
+        // transfer the memory
+
+        if (self.bytes) |source_ptr| {
+            const dest_ptr = first_slot;
+
+            @memcpy(dest_ptr, source_ptr, old_length);
+        }
+
+        // NOTE the newly added elements are left uninitialized
+
+        const result = RocList{
+            .dict_bytes = first_slot,
+            .length = new_length,
+        };
+
+        // NOTE we fuse an increment of all keys/values with a decrement of the input dict
+        utils.decref(allocator, alignment, self.bytes, old_length * element_width);
+
+        return result;
+    }
+};
+
+const Caller1 = fn (?[*]u8, ?[*]u8, ?[*]u8) callconv(.C) void;
+const Caller2 = fn (?[*]u8, ?[*]u8, ?[*]u8, ?[*]u8) callconv(.C) void;
+
+pub fn listMap(list: RocList, transform: Opaque, caller: Caller1, alignment: usize, old_element_width: usize, new_element_width: usize) callconv(.C) RocList {
+    if (list.bytes) |source_ptr| {
+        const size = list.len();
+        var i: usize = 0;
+        const output = RocList.allocate(std.heap.c_allocator, alignment, size, new_element_width);
+        const target_ptr = output.bytes orelse unreachable;
+
+        while (i < size) : (i += 1) {
+            caller(transform, source_ptr + (i * old_element_width), target_ptr + (i * new_element_width));
+        }
+
+        utils.decref(std.heap.c_allocator, alignment, list.bytes, size * old_element_width);
+
+        return output;
+    } else {
+        return RocList.empty();
+    }
+}
+
+pub fn listMapWithIndex(list: RocList, transform: Opaque, caller: Caller2, alignment: usize, old_element_width: usize, new_element_width: usize) callconv(.C) RocList {
+    if (list.bytes) |source_ptr| {
+        const size = list.len();
+        var i: usize = 0;
+        const output = RocList.allocate(std.heap.c_allocator, alignment, size, new_element_width);
+        const target_ptr = output.bytes orelse unreachable;
+
+        while (i < size) : (i += 1) {
+            caller(transform, @ptrCast(?[*]u8, &i), source_ptr + (i * old_element_width), target_ptr + (i * new_element_width));
+        }
+
+        utils.decref(std.heap.c_allocator, alignment, list.bytes, size * old_element_width);
+
+        return output;
+    } else {
+        return RocList.empty();
+    }
+}
+
+pub fn listKeepIf(list: RocList, transform: Opaque, caller: Caller1, alignment: usize, element_width: usize) callconv(.C) RocList {
+    if (list.bytes) |source_ptr| {
+        const size = list.len();
+        var i: usize = 0;
+        var output = RocList.allocate(std.heap.c_allocator, alignment, list.len(), list.len() * element_width);
+        const target_ptr = output.bytes orelse unreachable;
+
+        var kept: usize = 0;
+        while (i < size) : (i += 1) {
+            var keep = false;
+            const element = source_ptr + (i * element_width);
+            caller(transform, element, @ptrCast(?[*]u8, &keep));
+
+            if (keep) {
+                @memcpy(target_ptr + (kept * element_width), element, element_width);
+
+                kept += 1;
+            } else {
+                // TODO decrement the value?
+            }
+        }
+
+        output.length = kept;
+
+        utils.decref(std.heap.c_allocator, alignment, list.bytes, size * element_width);
+
+        return output;
+    } else {
+        return RocList.empty();
+    }
+}
+
+pub fn listKeepOks(list: RocList, transform: Opaque, caller: Caller1, alignment: usize, before_width: usize, result_width: usize, after_width: usize) callconv(.C) RocList {
+    return listKeepResult(list, RocResult.isOk, transform, caller, alignment, before_width, result_width, after_width);
+}
+
+pub fn listKeepErrs(list: RocList, transform: Opaque, caller: Caller1, alignment: usize, before_width: usize, result_width: usize, after_width: usize) callconv(.C) RocList {
+    return listKeepResult(list, RocResult.isErr, transform, caller, alignment, before_width, result_width, after_width);
+}
+
+pub fn listKeepResult(list: RocList, is_good_constructor: fn (RocResult) bool, transform: Opaque, caller: Caller1, alignment: usize, before_width: usize, result_width: usize, after_width: usize) RocList {
+    if (list.bytes) |source_ptr| {
+        const size = list.len();
+        var i: usize = 0;
+        var output = RocList.allocate(std.heap.c_allocator, alignment, list.len(), list.len() * after_width);
+        const target_ptr = output.bytes orelse unreachable;
+
+        var temporary = @ptrCast([*]u8, std.heap.c_allocator.alloc(u8, result_width) catch unreachable);
+
+        var kept: usize = 0;
+        while (i < size) : (i += 1) {
+            const element = source_ptr + (i * before_width);
+            caller(transform, element, temporary);
+
+            const result = utils.RocResult{ .bytes = temporary };
+
+            if (is_good_constructor(result)) {
+                @memcpy(target_ptr + (kept * after_width), temporary + @sizeOf(i64), after_width);
+
+                kept += 1;
+            }
+        }
+
+        output.length = kept;
+
+        utils.decref(std.heap.c_allocator, alignment, list.bytes, size * before_width);
+
+        return output;
+    } else {
+        return RocList.empty();
+    }
+}
+
+pub fn listWalk(list: RocList, stepper: Opaque, stepper_caller: Caller2, accum: Opaque, alignment: usize, element_width: usize, accum_width: usize, output: Opaque) callconv(.C) void {
+    if (accum_width == 0) {
+        return;
+    }
+
+    @memcpy(output orelse unreachable, accum orelse unreachable, accum_width);
+
+    if (list.bytes) |source_ptr| {
+        var i: usize = 0;
+        const size = list.len();
+        while (i < size) : (i += 1) {
+            const element = source_ptr + i * element_width;
+            stepper_caller(stepper, element, output, output);
+        }
+
+        const data_bytes = list.len() * element_width;
+        utils.decref(std.heap.c_allocator, alignment, list.bytes, data_bytes);
+    }
+}
+
+pub fn listWalkBackwards(list: RocList, stepper: Opaque, stepper_caller: Caller2, accum: Opaque, alignment: usize, element_width: usize, accum_width: usize, output: Opaque) callconv(.C) void {
+    if (accum_width == 0) {
+        return;
+    }
+
+    @memcpy(output orelse unreachable, accum orelse unreachable, accum_width);
+
+    if (list.bytes) |source_ptr| {
+        const size = list.len();
+        var i: usize = size;
+        while (i > 0) {
+            i -= 1;
+            const element = source_ptr + i * element_width;
+            stepper_caller(stepper, element, output, output);
+        }
+
+        const data_bytes = list.len() * element_width;
+        utils.decref(std.heap.c_allocator, alignment, list.bytes, data_bytes);
+    }
+}
+
+// List.contains : List k, k -> Bool
+pub fn listContains(list: RocList, key: Opaque, key_width: usize, is_eq: EqFn) callconv(.C) bool {
+    if (list.bytes) |source_ptr| {
+        const size = list.len();
+        var i: usize = 0;
+        while (i < size) : (i += 1) {
+            const element = source_ptr + i * key_width;
+            if (is_eq(element, key)) {
+                return true;
+            }
+        }
+    }
+
+    return false;
+}
diff --git a/compiler/builtins/bitcode/src/main.zig b/compiler/builtins/bitcode/src/main.zig
index 7bad53bdfa..1ecb482406 100644
--- a/compiler/builtins/bitcode/src/main.zig
+++ b/compiler/builtins/bitcode/src/main.zig
@@ -2,6 +2,20 @@ const builtin = @import("builtin");
 const std = @import("std");
 const testing = std.testing;
 
+// List Module
+const list = @import("list.zig");
+
+comptime {
+    exportListFn(list.listMap, "map");
+    exportListFn(list.listMapWithIndex, "map_with_index");
+    exportListFn(list.listKeepIf, "keep_if");
+    exportListFn(list.listWalk, "walk");
+    exportListFn(list.listWalkBackwards, "walk_backwards");
+    exportListFn(list.listKeepOks, "keep_oks");
+    exportListFn(list.listKeepErrs, "keep_errs");
+    exportListFn(list.listContains, "contains");
+}
+
 // Dict Module
 const dict = @import("dict.zig");
 const hash = @import("hash.zig");
@@ -16,6 +30,12 @@ comptime {
     exportDictFn(dict.elementsRc, "elementsRc");
     exportDictFn(dict.dictKeys, "keys");
     exportDictFn(dict.dictValues, "values");
+    exportDictFn(dict.dictUnion, "union");
+    exportDictFn(dict.dictIntersection, "intersection");
+    exportDictFn(dict.dictDifference, "difference");
+    exportDictFn(dict.dictWalk, "walk");
+
+    exportDictFn(dict.setFromList, "set_from_list");
 
     exportDictFn(hash.wyhash, "hash");
     exportDictFn(hash.wyhash_rocstr, "hash_str");
@@ -43,6 +63,7 @@ comptime {
     exportStrFn(str.strJoinWithC, "joinWith");
     exportStrFn(str.strNumberOfBytes, "number_of_bytes");
     exportStrFn(str.strFromIntC, "from_int");
+    exportStrFn(str.strFromFloatC, "from_float");
     exportStrFn(str.strEqual, "equal");
 }
 
@@ -60,6 +81,10 @@ fn exportDictFn(comptime func: anytype, comptime func_name: []const u8) void {
     exportBuiltinFn(func, "dict." ++ func_name);
 }
 
+fn exportListFn(comptime func: anytype, comptime func_name: []const u8) void {
+    exportBuiltinFn(func, "list." ++ func_name);
+}
+
 // Run all tests in imported modules
 // https://github.com/ziglang/zig/blob/master/lib/std/std.zig#L94
 test "" {
diff --git a/compiler/builtins/bitcode/src/str.zig b/compiler/builtins/bitcode/src/str.zig
index 16957835cd..97a4928ee3 100644
--- a/compiler/builtins/bitcode/src/str.zig
+++ b/compiler/builtins/bitcode/src/str.zig
@@ -302,6 +302,23 @@ fn strFromIntHelp(allocator: *Allocator, comptime T: type, int: T) RocStr {
     return RocStr.init(allocator, &buf, result.len);
 }
 
+// Str.fromFloat
+// When we actually use this in Roc, libc will be linked so we have access to std.heap.c_allocator
+pub fn strFromFloatC(float: f64) callconv(.C) RocStr {
+    // NOTE the compiled zig for float formatting seems to use LLVM11-specific features
+    // hopefully we can use zig instead of snprintf in the future when we upgrade
+    const c = @cImport({
+        // See https://github.com/ziglang/zig/issues/515
+        @cDefine("_NO_CRT_STDIO_INLINE", "1");
+        @cInclude("stdio.h");
+    });
+    var buf: [100]u8 = undefined;
+
+    const result = c.snprintf(&buf, 100, "%f", float);
+
+    return RocStr.init(std.heap.c_allocator, &buf, @intCast(usize, result));
+}
+
 // Str.split
 // When we actually use this in Roc, libc will be linked so we have access to std.heap.c_allocator
 pub fn strSplitInPlaceC(array: [*]RocStr, string: RocStr, delimiter: RocStr) callconv(.C) void {
diff --git a/compiler/builtins/bitcode/src/utils.zig b/compiler/builtins/bitcode/src/utils.zig
new file mode 100644
index 0000000000..e95f3ab711
--- /dev/null
+++ b/compiler/builtins/bitcode/src/utils.zig
@@ -0,0 +1,107 @@
+const std = @import("std");
+const Allocator = std.mem.Allocator;
+
+const REFCOUNT_ONE_ISIZE: comptime isize = std.math.minInt(isize);
+pub const REFCOUNT_ONE: usize = @bitCast(usize, REFCOUNT_ONE_ISIZE);
+
+pub fn decref(
+    allocator: *Allocator,
+    alignment: usize,
+    bytes_or_null: ?[*]u8,
+    data_bytes: usize,
+) void {
+    if (data_bytes == 0) {
+        return;
+    }
+
+    var bytes = bytes_or_null orelse return;
+
+    const usizes: [*]usize = @ptrCast([*]usize, @alignCast(8, bytes));
+
+    const refcount = (usizes - 1)[0];
+    const refcount_isize = @bitCast(isize, refcount);
+
+    switch (alignment) {
+        16 => {
+            if (refcount == REFCOUNT_ONE) {
+                allocator.free((bytes - 16)[0 .. 16 + data_bytes]);
+            } else if (refcount_isize < 0) {
+                (usizes - 1)[0] = refcount + 1;
+            }
+        },
+        else => {
+            // NOTE enums can currently have an alignment of < 8
+            if (refcount == REFCOUNT_ONE) {
+                allocator.free((bytes - 8)[0 .. 8 + data_bytes]);
+            } else if (refcount_isize < 0) {
+                (usizes - 1)[0] = refcount + 1;
+            }
+        },
+    }
+}
+
+pub fn allocateWithRefcount(
+    allocator: *Allocator,
+    alignment: usize,
+    data_bytes: usize,
+) [*]u8 {
+    comptime const result_in_place = false;
+
+    switch (alignment) {
+        16 => {
+            const length = 2 * @sizeOf(usize) + data_bytes;
+
+            var new_bytes: []align(16) u8 = allocator.alignedAlloc(u8, 16, length) catch unreachable;
+
+            var as_usize_array = @ptrCast([*]usize, new_bytes);
+            if (result_in_place) {
+                as_usize_array[0] = 0;
+                as_usize_array[1] = @intCast(usize, number_of_slots);
+            } else {
+                as_usize_array[0] = 0;
+                as_usize_array[1] = REFCOUNT_ONE;
+            }
+
+            var as_u8_array = @ptrCast([*]u8, new_bytes);
+            const first_slot = as_u8_array + 2 * @sizeOf(usize);
+
+            return first_slot;
+        },
+        else => {
+            const length = @sizeOf(usize) + data_bytes;
+
+            var new_bytes: []align(8) u8 = allocator.alignedAlloc(u8, 8, length) catch unreachable;
+
+            var as_usize_array = @ptrCast([*]usize, new_bytes);
+            if (result_in_place) {
+                as_usize_array[0] = @intCast(usize, number_of_slots);
+            } else {
+                as_usize_array[0] = REFCOUNT_ONE;
+            }
+
+            var as_u8_array = @ptrCast([*]u8, new_bytes);
+            const first_slot = as_u8_array + @sizeOf(usize);
+
+            return first_slot;
+        },
+    }
+}
+
+pub const RocResult = extern struct {
+    bytes: ?[*]u8,
+
+    pub fn isOk(self: RocResult) bool {
+        // assumptions
+        //
+        // - the tag is the first field
+        // - the tag is usize bytes wide
+        // - Ok has tag_id 1, because Err < Ok
+        const usizes: [*]usize = @ptrCast([*]usize, @alignCast(8, self.bytes));
+
+        return usizes[0] == 1;
+    }
+
+    pub fn isErr(self: RocResult) bool {
+        return !self.isOk();
+    }
+};
diff --git a/compiler/builtins/src/bitcode.rs b/compiler/builtins/src/bitcode.rs
index fc66f4c584..a220151bc9 100644
--- a/compiler/builtins/src/bitcode.rs
+++ b/compiler/builtins/src/bitcode.rs
@@ -38,6 +38,7 @@ pub const STR_STARTS_WITH: &str = "roc_builtins.str.starts_with";
 pub const STR_ENDS_WITH: &str = "roc_builtins.str.ends_with";
 pub const STR_NUMBER_OF_BYTES: &str = "roc_builtins.str.number_of_bytes";
 pub const STR_FROM_INT: &str = "roc_builtins.str.from_int";
+pub const STR_FROM_FLOAT: &str = "roc_builtins.str.from_float";
 pub const STR_EQUAL: &str = "roc_builtins.str.equal";
 
 pub const DICT_HASH: &str = "roc_builtins.dict.hash";
@@ -51,3 +52,18 @@ pub const DICT_GET: &str = "roc_builtins.dict.get";
 pub const DICT_ELEMENTS_RC: &str = "roc_builtins.dict.elementsRc";
 pub const DICT_KEYS: &str = "roc_builtins.dict.keys";
 pub const DICT_VALUES: &str = "roc_builtins.dict.values";
+pub const DICT_UNION: &str = "roc_builtins.dict.union";
+pub const DICT_DIFFERENCE: &str = "roc_builtins.dict.difference";
+pub const DICT_INTERSECTION: &str = "roc_builtins.dict.intersection";
+pub const DICT_WALK: &str = "roc_builtins.dict.walk";
+
+pub const SET_FROM_LIST: &str = "roc_builtins.dict.set_from_list";
+
+pub const LIST_MAP: &str = "roc_builtins.list.map";
+pub const LIST_MAP_WITH_INDEX: &str = "roc_builtins.list.map_with_index";
+pub const LIST_KEEP_IF: &str = "roc_builtins.list.keep_if";
+pub const LIST_KEEP_OKS: &str = "roc_builtins.list.keep_oks";
+pub const LIST_KEEP_ERRS: &str = "roc_builtins.list.keep_errs";
+pub const LIST_WALK: &str = "roc_builtins.list.walk";
+pub const LIST_WALK_BACKWARDS: &str = "roc_builtins.list.walk_backwards";
+pub const LIST_CONTAINS: &str = "roc_builtins.list.contains";
diff --git a/compiler/builtins/src/std.rs b/compiler/builtins/src/std.rs
index 7d66ed2d6c..42eb034581 100644
--- a/compiler/builtins/src/std.rs
+++ b/compiler/builtins/src/std.rs
@@ -10,6 +10,30 @@ use roc_types::solved_types::SolvedType;
 use roc_types::subs::VarId;
 use std::collections::HashMap;
 
+/// Example:
+///
+///     let_tvars! { a, b, c }
+///
+/// This is equivalent to:
+///
+///     let a = VarId::from_u32(1);
+///     let b = VarId::from_u32(2);
+///     let c = VarId::from_u32(3);
+///
+/// The idea is that this is less error-prone than assigning hardcoded IDs by hand.
+macro_rules! let_tvars {
+    ($($name:ident,)+) => { let_tvars!($($name),+) };
+    ($($name:ident),*) => {
+        let mut _current_tvar = 0;
+
+        $(
+            _current_tvar += 1;
+
+            let $name = VarId::from_u32(_current_tvar);
+        )*
+    };
+}
+
 #[derive(Clone, Copy, Debug)]
 pub enum Mode {
     Standard,
@@ -539,6 +563,12 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         top_level_function(vec![int_type(flex(TVAR1))], Box::new(str_type())),
     );
 
+    // fromFloat : Float a -> Str
+    add_type(
+        Symbol::STR_FROM_FLOAT,
+        top_level_function(vec![float_type(flex(TVAR1))], Box::new(str_type())),
+    );
+
     // List module
 
     // get : List elem, Nat -> Result elem [ OutOfBounds ]*
@@ -652,6 +682,38 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         ),
     );
 
+    // keepOks : List before, (before -> Result after *) -> List after
+    add_type(Symbol::LIST_KEEP_OKS, {
+        let_tvars! { star, cvar, before, after};
+        top_level_function(
+            vec![
+                list_type(flex(before)),
+                closure(
+                    vec![flex(before)],
+                    cvar,
+                    Box::new(result_type(flex(after), flex(star))),
+                ),
+            ],
+            Box::new(list_type(flex(after))),
+        )
+    });
+
+    // keepOks : List before, (before -> Result * after) -> List after
+    add_type(Symbol::LIST_KEEP_ERRS, {
+        let_tvars! { star, cvar, before, after};
+        top_level_function(
+            vec![
+                list_type(flex(before)),
+                closure(
+                    vec![flex(before)],
+                    cvar,
+                    Box::new(result_type(flex(star), flex(after))),
+                ),
+            ],
+            Box::new(list_type(flex(after))),
+        )
+    });
+
     // map : List before, (before -> after) -> List after
     add_type(
         Symbol::LIST_MAP,
@@ -664,6 +726,18 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         ),
     );
 
+    // mapWithIndex : List before, (Nat, before -> after) -> List after
+    add_type(Symbol::LIST_MAP_WITH_INDEX, {
+        let_tvars! { cvar, before, after};
+        top_level_function(
+            vec![
+                list_type(flex(before)),
+                closure(vec![nat_type(), flex(before)], cvar, Box::new(flex(after))),
+            ],
+            Box::new(list_type(flex(after))),
+        )
+    });
+
     // append : List elem, elem -> List elem
     add_type(
         Symbol::LIST_APPEND,
@@ -819,6 +893,59 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         ),
     );
 
+    // Dict.union : Dict k v, Dict k v -> Dict k v
+    add_type(
+        Symbol::DICT_UNION,
+        top_level_function(
+            vec![
+                dict_type(flex(TVAR1), flex(TVAR2)),
+                dict_type(flex(TVAR1), flex(TVAR2)),
+            ],
+            Box::new(dict_type(flex(TVAR1), flex(TVAR2))),
+        ),
+    );
+
+    // Dict.intersection : Dict k v, Dict k v -> Dict k v
+    add_type(
+        Symbol::DICT_INTERSECTION,
+        top_level_function(
+            vec![
+                dict_type(flex(TVAR1), flex(TVAR2)),
+                dict_type(flex(TVAR1), flex(TVAR2)),
+            ],
+            Box::new(dict_type(flex(TVAR1), flex(TVAR2))),
+        ),
+    );
+
+    // Dict.difference : Dict k v, Dict k v -> Dict k v
+    add_type(
+        Symbol::DICT_DIFFERENCE,
+        top_level_function(
+            vec![
+                dict_type(flex(TVAR1), flex(TVAR2)),
+                dict_type(flex(TVAR1), flex(TVAR2)),
+            ],
+            Box::new(dict_type(flex(TVAR1), flex(TVAR2))),
+        ),
+    );
+
+    // Dict.walk : Dict k v, (k, v, accum -> accum), accum -> accum
+    add_type(
+        Symbol::DICT_WALK,
+        top_level_function(
+            vec![
+                dict_type(flex(TVAR1), flex(TVAR2)),
+                closure(
+                    vec![flex(TVAR1), flex(TVAR2), flex(TVAR3)],
+                    TVAR4,
+                    Box::new(flex(TVAR3)),
+                ),
+                flex(TVAR3),
+            ],
+            Box::new(flex(TVAR3)),
+        ),
+    );
+
     // Set module
 
     // empty : Set a
@@ -830,6 +957,30 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         top_level_function(vec![flex(TVAR1)], Box::new(set_type(flex(TVAR1)))),
     );
 
+    // len : Set * -> Nat
+    add_type(
+        Symbol::SET_LEN,
+        top_level_function(vec![set_type(flex(TVAR1))], Box::new(nat_type())),
+    );
+
+    // toList : Set a -> List a
+    add_type(
+        Symbol::SET_TO_LIST,
+        top_level_function(
+            vec![set_type(flex(TVAR1))],
+            Box::new(list_type(flex(TVAR1))),
+        ),
+    );
+
+    // fromList : Set a -> List a
+    add_type(
+        Symbol::SET_FROM_LIST,
+        top_level_function(
+            vec![list_type(flex(TVAR1))],
+            Box::new(set_type(flex(TVAR1))),
+        ),
+    );
+
     // union : Set a, Set a -> Set a
     add_type(
         Symbol::SET_UNION,
@@ -839,18 +990,27 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         ),
     );
 
-    // diff : Set a, Set a -> Set a
+    // difference : Set a, Set a -> Set a
     add_type(
-        Symbol::SET_DIFF,
+        Symbol::SET_DIFFERENCE,
         top_level_function(
             vec![set_type(flex(TVAR1)), set_type(flex(TVAR1))],
             Box::new(set_type(flex(TVAR1))),
         ),
     );
 
-    // foldl : Set a, (a -> b -> b), b -> b
+    // intersection : Set a, Set a -> Set a
     add_type(
-        Symbol::SET_FOLDL,
+        Symbol::SET_INTERSECTION,
+        top_level_function(
+            vec![set_type(flex(TVAR1)), set_type(flex(TVAR1))],
+            Box::new(set_type(flex(TVAR1))),
+        ),
+    );
+
+    // Set.walk : Set a, (a, b -> b), b -> b
+    add_type(
+        Symbol::SET_WALK,
         top_level_function(
             vec![
                 set_type(flex(TVAR1)),
@@ -877,6 +1037,14 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         ),
     );
 
+    add_type(
+        Symbol::SET_CONTAINS,
+        top_level_function(
+            vec![set_type(flex(TVAR1)), flex(TVAR1)],
+            Box::new(bool_type()),
+        ),
+    );
+
     // Result module
 
     // map : Result a err, (a -> b) -> Result b err
@@ -891,6 +1059,27 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         ),
     );
 
+    // mapErr : Result a x, (x -> y) -> Result a x
+    add_type(
+        Symbol::RESULT_MAP_ERR,
+        top_level_function(
+            vec![
+                result_type(flex(TVAR1), flex(TVAR3)),
+                closure(vec![flex(TVAR3)], TVAR4, Box::new(flex(TVAR2))),
+            ],
+            Box::new(result_type(flex(TVAR1), flex(TVAR2))),
+        ),
+    );
+
+    // withDefault : Result a x, a -> a
+    add_type(
+        Symbol::RESULT_WITH_DEFAULT,
+        top_level_function(
+            vec![result_type(flex(TVAR1), flex(TVAR3)), flex(TVAR1)],
+            Box::new(flex(TVAR1)),
+        ),
+    );
+
     types
 }
 
diff --git a/compiler/builtins/src/unique.rs b/compiler/builtins/src/unique.rs
index f2a4034465..6ffad680e0 100644
--- a/compiler/builtins/src/unique.rs
+++ b/compiler/builtins/src/unique.rs
@@ -1053,13 +1053,13 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
     // diff : Attr * (Set * a)
     //      , Attr * (Set * a)
     //     -> Attr * (Set * a)
-    add_type(Symbol::SET_DIFF, set_combine);
+    add_type(Symbol::SET_DIFFERENCE, set_combine);
 
     // foldl : Attr (* | u) (Set (Attr u a))
     //       , Attr Shared (Attr u a -> b -> b)
     //       , b
     //      -> b
-    add_type(Symbol::SET_FOLDL, {
+    add_type(Symbol::SET_WALK, {
         let_tvars! { star, u, a, b, closure };
 
         unique_function(
diff --git a/compiler/can/src/builtins.rs b/compiler/can/src/builtins.rs
index c62d0a37c9..2865f31dff 100644
--- a/compiler/can/src/builtins.rs
+++ b/compiler/can/src/builtins.rs
@@ -1,10 +1,11 @@
 use crate::def::Def;
 use crate::expr::Expr::*;
-use crate::expr::{Expr, Recursive};
+use crate::expr::{Expr, Recursive, WhenBranch};
 use crate::pattern::Pattern;
 use roc_collections::all::{MutMap, SendMap};
 use roc_module::ident::TagName;
 use roc_module::low_level::LowLevel;
+use roc_module::operator::CalledVia;
 use roc_module::symbol::Symbol;
 use roc_region::all::{Located, Region};
 use roc_types::subs::{VarStore, Variable};
@@ -60,6 +61,7 @@ pub fn builtin_defs_map(symbol: Symbol, var_store: &mut VarStore) -> Option<Def>
         STR_ENDS_WITH => str_ends_with,
         STR_COUNT_GRAPHEMES => str_count_graphemes,
         STR_FROM_INT => str_from_int,
+        STR_FROM_FLOAT=> str_from_float,
         LIST_LEN => list_len,
         LIST_GET => list_get,
         LIST_SET => list_set,
@@ -76,18 +78,38 @@ pub fn builtin_defs_map(symbol: Symbol, var_store: &mut VarStore) -> Option<Def>
         LIST_PREPEND => list_prepend,
         LIST_JOIN => list_join,
         LIST_MAP => list_map,
+        LIST_MAP_WITH_INDEX => list_map_with_index,
         LIST_KEEP_IF => list_keep_if,
+        LIST_KEEP_OKS => list_keep_oks,
+        LIST_KEEP_ERRS=> list_keep_errs,
         LIST_WALK => list_walk,
         LIST_WALK_BACKWARDS => list_walk_backwards,
         DICT_TEST_HASH => dict_hash_test_only,
         DICT_LEN => dict_len,
         DICT_EMPTY => dict_empty,
+        DICT_SINGLETON => dict_singleton,
         DICT_INSERT => dict_insert,
         DICT_REMOVE => dict_remove,
         DICT_GET => dict_get,
         DICT_CONTAINS => dict_contains,
         DICT_KEYS => dict_keys,
         DICT_VALUES => dict_values,
+        DICT_UNION=> dict_union,
+        DICT_INTERSECTION=> dict_intersection,
+        DICT_DIFFERENCE=> dict_difference,
+        DICT_WALK=> dict_walk,
+        SET_EMPTY => set_empty,
+        SET_LEN => set_len,
+        SET_SINGLETON => set_singleton,
+        SET_UNION=> set_union,
+        SET_INTERSECTION => set_intersection,
+        SET_DIFFERENCE => set_difference,
+        SET_TO_LIST => set_to_list,
+        SET_FROM_LIST => set_from_list,
+        SET_INSERT => set_insert,
+        SET_REMOVE => set_remove,
+        SET_CONTAINS => set_contains,
+        SET_WALK=> set_walk,
         NUM_ADD => num_add,
         NUM_ADD_CHECKED => num_add_checked,
         NUM_ADD_WRAP => num_add_wrap,
@@ -128,7 +150,10 @@ pub fn builtin_defs_map(symbol: Symbol, var_store: &mut VarStore) -> Option<Def>
         NUM_MAX_INT => num_max_int,
         NUM_MIN_INT => num_min_int,
         NUM_BITWISE_AND => num_bitwise_and,
-        NUM_BITWISE_XOR => num_bitwise_xor
+        NUM_BITWISE_XOR => num_bitwise_xor,
+        RESULT_MAP => result_map,
+        RESULT_MAP_ERR => result_map_err,
+        RESULT_WITH_DEFAULT => result_with_default,
     }
 }
 
@@ -164,6 +189,7 @@ pub fn builtin_defs(var_store: &mut VarStore) -> MutMap<Symbol, Def> {
         Symbol::STR_ENDS_WITH => str_ends_with,
         Symbol::STR_COUNT_GRAPHEMES => str_count_graphemes,
         Symbol::STR_FROM_INT => str_from_int,
+        Symbol::STR_FROM_FLOAT=> str_from_float,
         Symbol::LIST_LEN => list_len,
         Symbol::LIST_GET => list_get,
         Symbol::LIST_SET => list_set,
@@ -180,18 +206,38 @@ pub fn builtin_defs(var_store: &mut VarStore) -> MutMap<Symbol, Def> {
         Symbol::LIST_PREPEND => list_prepend,
         Symbol::LIST_JOIN => list_join,
         Symbol::LIST_MAP => list_map,
+        Symbol::LIST_MAP_WITH_INDEX => list_map_with_index,
         Symbol::LIST_KEEP_IF => list_keep_if,
+        Symbol::LIST_KEEP_OKS => list_keep_oks,
+        Symbol::LIST_KEEP_ERRS=> list_keep_errs,
         Symbol::LIST_WALK => list_walk,
         Symbol::LIST_WALK_BACKWARDS => list_walk_backwards,
         Symbol::DICT_TEST_HASH => dict_hash_test_only,
         Symbol::DICT_LEN => dict_len,
         Symbol::DICT_EMPTY => dict_empty,
+        Symbol::DICT_SINGLETON => dict_singleton,
         Symbol::DICT_INSERT => dict_insert,
         Symbol::DICT_REMOVE => dict_remove,
         Symbol::DICT_GET => dict_get,
         Symbol::DICT_CONTAINS => dict_contains,
         Symbol::DICT_KEYS => dict_keys,
         Symbol::DICT_VALUES => dict_values,
+        Symbol::DICT_UNION=> dict_union,
+        Symbol::DICT_INTERSECTION=> dict_intersection,
+        Symbol::DICT_DIFFERENCE=> dict_difference,
+        Symbol::DICT_WALK=> dict_walk,
+        Symbol::SET_EMPTY => set_empty,
+        Symbol::SET_LEN => set_len,
+        Symbol::SET_SINGLETON => set_singleton,
+        Symbol::SET_UNION=> set_union,
+        Symbol::SET_INTERSECTION=> set_intersection,
+        Symbol::SET_DIFFERENCE=> set_difference,
+        Symbol::SET_TO_LIST => set_to_list,
+        Symbol::SET_FROM_LIST => set_from_list,
+        Symbol::SET_INSERT => set_insert,
+        Symbol::SET_REMOVE => set_remove,
+        Symbol::SET_CONTAINS => set_contains,
+        Symbol::SET_WALK => set_walk,
         Symbol::NUM_ADD => num_add,
         Symbol::NUM_ADD_CHECKED => num_add_checked,
         Symbol::NUM_ADD_WRAP => num_add_wrap,
@@ -227,9 +273,83 @@ pub fn builtin_defs(var_store: &mut VarStore) -> MutMap<Symbol, Def> {
         Symbol::NUM_ASIN => num_asin,
         Symbol::NUM_MAX_INT => num_max_int,
         Symbol::NUM_MIN_INT => num_min_int,
+        Symbol::RESULT_MAP => result_map,
+        Symbol::RESULT_MAP_ERR => result_map_err,
+        Symbol::RESULT_WITH_DEFAULT => result_with_default,
     }
 }
 
+fn lowlevel_1(symbol: Symbol, op: LowLevel, var_store: &mut VarStore) -> Def {
+    let arg1_var = var_store.fresh();
+    let ret_var = var_store.fresh();
+
+    let body = RunLowLevel {
+        op,
+        args: vec![(arg1_var, Var(Symbol::ARG_1))],
+        ret_var,
+    };
+
+    defn(
+        symbol,
+        vec![(arg1_var, Symbol::ARG_1)],
+        var_store,
+        body,
+        ret_var,
+    )
+}
+
+fn lowlevel_2(symbol: Symbol, op: LowLevel, var_store: &mut VarStore) -> Def {
+    let arg1_var = var_store.fresh();
+    let arg2_var = var_store.fresh();
+    let ret_var = var_store.fresh();
+
+    let body = RunLowLevel {
+        op,
+        args: vec![
+            (arg1_var, Var(Symbol::ARG_1)),
+            (arg2_var, Var(Symbol::ARG_2)),
+        ],
+        ret_var,
+    };
+
+    defn(
+        symbol,
+        vec![(arg1_var, Symbol::ARG_1), (arg2_var, Symbol::ARG_2)],
+        var_store,
+        body,
+        ret_var,
+    )
+}
+
+fn lowlevel_3(symbol: Symbol, op: LowLevel, var_store: &mut VarStore) -> Def {
+    let arg1_var = var_store.fresh();
+    let arg2_var = var_store.fresh();
+    let arg3_var = var_store.fresh();
+    let ret_var = var_store.fresh();
+
+    let body = RunLowLevel {
+        op,
+        args: vec![
+            (arg1_var, Var(Symbol::ARG_1)),
+            (arg2_var, Var(Symbol::ARG_2)),
+            (arg3_var, Var(Symbol::ARG_3)),
+        ],
+        ret_var,
+    };
+
+    defn(
+        symbol,
+        vec![
+            (arg1_var, Symbol::ARG_1),
+            (arg2_var, Symbol::ARG_2),
+            (arg3_var, Symbol::ARG_3),
+        ],
+        var_store,
+        body,
+        ret_var,
+    )
+}
+
 /// Num.maxInt : Int
 fn num_max_int(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let int_var = var_store.fresh();
@@ -1372,7 +1492,7 @@ fn str_count_graphemes(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
-/// Str.fromInt : Int -> Str
+/// Str.fromInt : Int * -> Str
 fn str_from_int(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let int_var = var_store.fresh();
     let str_var = var_store.fresh();
@@ -1392,6 +1512,26 @@ fn str_from_int(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
+/// Str.fromFloat : Float * -> Str
+fn str_from_float(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let float_var = var_store.fresh();
+    let str_var = var_store.fresh();
+
+    let body = RunLowLevel {
+        op: LowLevel::StrFromFloat,
+        args: vec![(float_var, Var(Symbol::ARG_1))],
+        ret_var: str_var,
+    };
+
+    defn(
+        symbol,
+        vec![(float_var, Symbol::ARG_1)],
+        var_store,
+        body,
+        str_var,
+    )
+}
+
 /// List.concat : List elem, List elem -> List elem
 fn list_concat(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let list_var = var_store.fresh();
@@ -1800,94 +1940,37 @@ fn list_keep_if(symbol: Symbol, var_store: &mut VarStore) -> Def {
 
 /// List.contains : List elem, elem -> Bool
 fn list_contains(symbol: Symbol, var_store: &mut VarStore) -> Def {
-    let list_var = var_store.fresh();
-    let elem_var = var_store.fresh();
-    let bool_var = var_store.fresh();
+    lowlevel_2(symbol, LowLevel::ListContains, var_store)
+}
 
-    let body = RunLowLevel {
-        op: LowLevel::ListContains,
-        args: vec![
-            (list_var, Var(Symbol::ARG_1)),
-            (elem_var, Var(Symbol::ARG_2)),
-        ],
-        ret_var: bool_var,
-    };
+/// List.keepOks : List before, (before -> Result after *) -> List after
+fn list_keep_oks(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_2(symbol, LowLevel::ListKeepOks, var_store)
+}
 
-    defn(
-        symbol,
-        vec![(list_var, Symbol::ARG_1), (elem_var, Symbol::ARG_2)],
-        var_store,
-        body,
-        bool_var,
-    )
+/// List.keepErrs: List before, (before -> Result * after) -> List after
+fn list_keep_errs(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_2(symbol, LowLevel::ListKeepErrs, var_store)
 }
 
 /// List.map : List before, (before -> after) -> List after
 fn list_map(symbol: Symbol, var_store: &mut VarStore) -> Def {
-    let list_var = var_store.fresh();
-    let func_var = var_store.fresh();
-    let ret_list_var = var_store.fresh();
+    lowlevel_2(symbol, LowLevel::ListMap, var_store)
+}
 
-    let body = RunLowLevel {
-        op: LowLevel::ListMap,
-        args: vec![
-            (list_var, Var(Symbol::ARG_1)),
-            (func_var, Var(Symbol::ARG_2)),
-        ],
-        ret_var: ret_list_var,
-    };
-
-    defn(
-        symbol,
-        vec![(list_var, Symbol::ARG_1), (func_var, Symbol::ARG_2)],
-        var_store,
-        body,
-        ret_list_var,
-    )
+/// List.mapWithIndex : List before, (Nat, before -> after) -> List after
+fn list_map_with_index(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_2(symbol, LowLevel::ListMapWithIndex, var_store)
 }
 
 /// Dict.hashTestOnly : k, v -> Nat
 pub fn dict_hash_test_only(symbol: Symbol, var_store: &mut VarStore) -> Def {
-    let key_var = var_store.fresh();
-    let value_var = var_store.fresh();
-    let nat_var = var_store.fresh();
-
-    let body = RunLowLevel {
-        op: LowLevel::Hash,
-        args: vec![
-            (key_var, Var(Symbol::ARG_1)),
-            (value_var, Var(Symbol::ARG_2)),
-        ],
-        ret_var: nat_var,
-    };
-
-    defn(
-        symbol,
-        vec![(key_var, Symbol::ARG_1), (value_var, Symbol::ARG_2)],
-        var_store,
-        body,
-        nat_var,
-    )
+    lowlevel_2(symbol, LowLevel::Hash, var_store)
 }
 
 /// Dict.len : Dict * * -> Nat
 fn dict_len(symbol: Symbol, var_store: &mut VarStore) -> Def {
-    let size_var = var_store.fresh();
-    let dict_var = var_store.fresh();
-
-    let body = RunLowLevel {
-        op: LowLevel::DictSize,
-        args: vec![(dict_var, Var(Symbol::ARG_1))],
-        ret_var: size_var,
-    };
-
-    defn(
-        symbol,
-        vec![(dict_var, Symbol::ARG_1)],
-        var_store,
-        body,
-        size_var,
-    )
+    lowlevel_1(symbol, LowLevel::DictSize, var_store)
 }
 
 /// Dict.empty : Dict * *
@@ -1908,80 +1991,50 @@ fn dict_empty(symbol: Symbol, var_store: &mut VarStore) -> Def {
     }
 }
 
-/// Dict.insert : Dict k v, k, v -> Dict k v
-fn dict_insert(symbol: Symbol, var_store: &mut VarStore) -> Def {
-    let dict_var = var_store.fresh();
+/// Dict.singleton : k, v -> Dict k v
+fn dict_singleton(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let key_var = var_store.fresh();
-    let val_var = var_store.fresh();
+    let value_var = var_store.fresh();
+    let dict_var = var_store.fresh();
+
+    let empty = RunLowLevel {
+        op: LowLevel::DictEmpty,
+        args: vec![],
+        ret_var: dict_var,
+    };
 
     let body = RunLowLevel {
         op: LowLevel::DictInsert,
         args: vec![
-            (dict_var, Var(Symbol::ARG_1)),
-            (key_var, Var(Symbol::ARG_2)),
-            (val_var, Var(Symbol::ARG_3)),
+            (dict_var, empty),
+            (key_var, Var(Symbol::ARG_1)),
+            (value_var, Var(Symbol::ARG_2)),
         ],
         ret_var: dict_var,
     };
 
     defn(
         symbol,
-        vec![
-            (dict_var, Symbol::ARG_1),
-            (key_var, Symbol::ARG_2),
-            (val_var, Symbol::ARG_3),
-        ],
+        vec![(key_var, Symbol::ARG_1), (value_var, Symbol::ARG_2)],
         var_store,
         body,
         dict_var,
     )
 }
 
+/// Dict.insert : Dict k v, k, v -> Dict k v
+fn dict_insert(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_3(symbol, LowLevel::DictInsert, var_store)
+}
+
 /// Dict.remove : Dict k v, k -> Dict k v
 fn dict_remove(symbol: Symbol, var_store: &mut VarStore) -> Def {
-    let dict_var = var_store.fresh();
-    let key_var = var_store.fresh();
-
-    let body = RunLowLevel {
-        op: LowLevel::DictRemove,
-        args: vec![
-            (dict_var, Var(Symbol::ARG_1)),
-            (key_var, Var(Symbol::ARG_2)),
-        ],
-        ret_var: dict_var,
-    };
-
-    defn(
-        symbol,
-        vec![(dict_var, Symbol::ARG_1), (key_var, Symbol::ARG_2)],
-        var_store,
-        body,
-        dict_var,
-    )
+    lowlevel_2(symbol, LowLevel::DictRemove, var_store)
 }
 
 /// Dict.contains : Dict k v, k -> Bool
 fn dict_contains(symbol: Symbol, var_store: &mut VarStore) -> Def {
-    let dict_var = var_store.fresh();
-    let key_var = var_store.fresh();
-    let bool_var = var_store.fresh();
-
-    let body = RunLowLevel {
-        op: LowLevel::DictContains,
-        args: vec![
-            (dict_var, Var(Symbol::ARG_1)),
-            (key_var, Var(Symbol::ARG_2)),
-        ],
-        ret_var: bool_var,
-    };
-
-    defn(
-        symbol,
-        vec![(dict_var, Symbol::ARG_1), (key_var, Symbol::ARG_2)],
-        var_store,
-        body,
-        bool_var,
-    )
+    lowlevel_2(symbol, LowLevel::DictContains, var_store)
 }
 
 /// Dict.get : Dict k v, k -> Result v [ KeyNotFound ]*
@@ -2067,41 +2120,208 @@ fn dict_get(symbol: Symbol, var_store: &mut VarStore) -> Def {
 
 /// Dict.keys : Dict k v -> List k
 fn dict_keys(symbol: Symbol, var_store: &mut VarStore) -> Def {
-    let dict_var = var_store.fresh();
-    let list_var = var_store.fresh();
-
-    let body = RunLowLevel {
-        op: LowLevel::DictKeys,
-        args: vec![(dict_var, Var(Symbol::ARG_1))],
-        ret_var: list_var,
-    };
-
-    defn(
-        symbol,
-        vec![(dict_var, Symbol::ARG_1)],
-        var_store,
-        body,
-        list_var,
-    )
+    lowlevel_1(symbol, LowLevel::DictKeys, var_store)
 }
 
 /// Dict.values : Dict k v -> List v
 fn dict_values(symbol: Symbol, var_store: &mut VarStore) -> Def {
-    let dict_var = var_store.fresh();
-    let list_var = var_store.fresh();
+    lowlevel_1(symbol, LowLevel::DictValues, var_store)
+}
+
+/// Dict.union : Dict k v, Dict k v -> Dict k v
+fn dict_union(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_2(symbol, LowLevel::DictUnion, var_store)
+}
+
+/// Dict.difference : Dict k v, Dict k v -> Dict k v
+fn dict_difference(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_2(symbol, LowLevel::DictDifference, var_store)
+}
+
+/// Dict.intersection : Dict k v, Dict k v -> Dict k v
+fn dict_intersection(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_2(symbol, LowLevel::DictIntersection, var_store)
+}
+
+/// Dict.walk : Dict k v, (k, v, accum -> accum), accum -> accum
+fn dict_walk(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_3(symbol, LowLevel::DictWalk, var_store)
+}
+
+/// Set.empty : Set *
+fn set_empty(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let set_var = var_store.fresh();
+    let body = RunLowLevel {
+        op: LowLevel::DictEmpty,
+        args: vec![],
+        ret_var: set_var,
+    };
+
+    Def {
+        annotation: None,
+        expr_var: set_var,
+        loc_expr: Located::at_zero(body),
+        loc_pattern: Located::at_zero(Pattern::Identifier(symbol)),
+        pattern_vars: SendMap::default(),
+    }
+}
+
+/// Set.singleton : k -> Set k
+fn set_singleton(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let key_var = var_store.fresh();
+    let set_var = var_store.fresh();
+    let value_var = Variable::EMPTY_RECORD;
+
+    let empty = RunLowLevel {
+        op: LowLevel::DictEmpty,
+        args: vec![],
+        ret_var: set_var,
+    };
 
     let body = RunLowLevel {
-        op: LowLevel::DictValues,
-        args: vec![(dict_var, Var(Symbol::ARG_1))],
-        ret_var: list_var,
+        op: LowLevel::DictInsert,
+        args: vec![
+            (set_var, empty),
+            (key_var, Var(Symbol::ARG_1)),
+            (value_var, EmptyRecord),
+        ],
+        ret_var: set_var,
     };
 
     defn(
         symbol,
-        vec![(dict_var, Symbol::ARG_1)],
+        vec![(key_var, Symbol::ARG_1)],
         var_store,
         body,
-        list_var,
+        set_var,
+    )
+}
+
+/// Set.len : Set * -> Nat
+fn set_len(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_1(symbol, LowLevel::DictSize, var_store)
+}
+
+/// Dict.union : Dict k v, Dict k v -> Dict k v
+fn set_union(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_2(symbol, LowLevel::DictUnion, var_store)
+}
+
+/// Dict.difference : Dict k v, Dict k v -> Dict k v
+fn set_difference(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_2(symbol, LowLevel::DictDifference, var_store)
+}
+
+/// Dict.intersection : Dict k v, Dict k v -> Dict k v
+fn set_intersection(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_2(symbol, LowLevel::DictIntersection, var_store)
+}
+
+/// Set.toList : Set k -> List k
+fn set_to_list(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    dict_keys(symbol, var_store)
+}
+
+/// Set.fromList : List k -> Set k
+fn set_from_list(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_1(symbol, LowLevel::SetFromList, var_store)
+}
+
+/// Set.insert : Set k, k -> Set k
+fn set_insert(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let dict_var = var_store.fresh();
+    let key_var = var_store.fresh();
+    let val_var = Variable::EMPTY_RECORD;
+
+    let body = RunLowLevel {
+        op: LowLevel::DictInsert,
+        args: vec![
+            (dict_var, Var(Symbol::ARG_1)),
+            (key_var, Var(Symbol::ARG_2)),
+            (val_var, EmptyRecord),
+        ],
+        ret_var: dict_var,
+    };
+
+    defn(
+        symbol,
+        vec![(dict_var, Symbol::ARG_1), (key_var, Symbol::ARG_2)],
+        var_store,
+        body,
+        dict_var,
+    )
+}
+
+/// Set.remove : Set k, k -> Set k
+fn set_remove(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    dict_remove(symbol, var_store)
+}
+
+/// Set.remove : Set k, k -> Set k
+fn set_contains(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    dict_contains(symbol, var_store)
+}
+
+/// Set.walk : Set k,  (k, accum -> accum), accum -> accum
+fn set_walk(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let dict_var = var_store.fresh();
+    let func_var = var_store.fresh();
+    let key_var = var_store.fresh();
+    let accum_var = var_store.fresh();
+    let wrapper_var = var_store.fresh();
+
+    let user_function = Box::new((
+        func_var,
+        no_region(Var(Symbol::ARG_2)),
+        var_store.fresh(),
+        accum_var,
+    ));
+
+    let call_func = Call(
+        user_function,
+        vec![
+            (key_var, no_region(Var(Symbol::ARG_5))),
+            (accum_var, no_region(Var(Symbol::ARG_6))),
+        ],
+        CalledVia::Space,
+    );
+
+    let wrapper = Closure {
+        function_type: wrapper_var,
+        closure_type: var_store.fresh(),
+        closure_ext_var: var_store.fresh(),
+        return_type: accum_var,
+        name: Symbol::SET_WALK_USER_FUNCTION,
+        recursive: Recursive::NotRecursive,
+        captured_symbols: vec![(Symbol::ARG_2, func_var)],
+        arguments: vec![
+            (key_var, no_region(Pattern::Identifier(Symbol::ARG_5))),
+            (Variable::EMPTY_RECORD, no_region(Pattern::Underscore)),
+            (accum_var, no_region(Pattern::Identifier(Symbol::ARG_6))),
+        ],
+        loc_body: Box::new(no_region(call_func)),
+    };
+
+    let body = RunLowLevel {
+        op: LowLevel::DictWalk,
+        args: vec![
+            (dict_var, Var(Symbol::ARG_1)),
+            (wrapper_var, wrapper),
+            (accum_var, Var(Symbol::ARG_3)),
+        ],
+        ret_var: accum_var,
+    };
+
+    defn(
+        symbol,
+        vec![
+            (dict_var, Symbol::ARG_1),
+            (func_var, Symbol::ARG_2),
+            (accum_var, Symbol::ARG_3),
+        ],
+        var_store,
+        body,
+        accum_var,
     )
 }
 
@@ -2511,6 +2731,263 @@ fn list_last(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
+fn result_map(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let ret_var = var_store.fresh();
+    let func_var = var_store.fresh();
+    let result_var = var_store.fresh();
+
+    let mut branches = vec![];
+
+    {
+        let user_function = Box::new((
+            func_var,
+            no_region(Var(Symbol::ARG_2)),
+            var_store.fresh(),
+            var_store.fresh(),
+        ));
+
+        let call_func = Call(
+            user_function,
+            vec![(var_store.fresh(), no_region(Var(Symbol::ARG_5)))],
+            CalledVia::Space,
+        );
+
+        let tag_name = TagName::Global("Ok".into());
+
+        // ok branch
+        let ok = Tag {
+            variant_var: var_store.fresh(),
+            ext_var: var_store.fresh(),
+            name: tag_name.clone(),
+            arguments: vec![(var_store.fresh(), no_region(call_func))],
+        };
+
+        let pattern = Pattern::AppliedTag {
+            whole_var: result_var,
+            ext_var: var_store.fresh(),
+            tag_name,
+            arguments: vec![(
+                var_store.fresh(),
+                no_region(Pattern::Identifier(Symbol::ARG_5)),
+            )],
+        };
+
+        let branch = WhenBranch {
+            patterns: vec![no_region(pattern)],
+            value: no_region(ok),
+            guard: None,
+        };
+
+        branches.push(branch);
+    }
+
+    {
+        // err branch
+        let tag_name = TagName::Global("Err".into());
+
+        let err = Tag {
+            variant_var: var_store.fresh(),
+            ext_var: var_store.fresh(),
+            name: tag_name.clone(),
+            arguments: vec![(var_store.fresh(), no_region(Var(Symbol::ARG_4)))],
+        };
+
+        let pattern = Pattern::AppliedTag {
+            whole_var: result_var,
+            ext_var: var_store.fresh(),
+            tag_name,
+            arguments: vec![(
+                var_store.fresh(),
+                no_region(Pattern::Identifier(Symbol::ARG_4)),
+            )],
+        };
+
+        let branch = WhenBranch {
+            patterns: vec![no_region(pattern)],
+            value: no_region(err),
+            guard: None,
+        };
+
+        branches.push(branch);
+    }
+
+    let body = When {
+        cond_var: result_var,
+        expr_var: ret_var,
+        region: Region::zero(),
+        loc_cond: Box::new(no_region(Var(Symbol::ARG_1))),
+        branches,
+    };
+
+    defn(
+        symbol,
+        vec![(result_var, Symbol::ARG_1), (func_var, Symbol::ARG_2)],
+        var_store,
+        body,
+        ret_var,
+    )
+}
+
+fn result_map_err(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let ret_var = var_store.fresh();
+    let func_var = var_store.fresh();
+    let result_var = var_store.fresh();
+
+    let mut branches = vec![];
+
+    {
+        let user_function = Box::new((
+            func_var,
+            no_region(Var(Symbol::ARG_2)),
+            var_store.fresh(),
+            var_store.fresh(),
+        ));
+
+        let call_func = Call(
+            user_function,
+            vec![(var_store.fresh(), no_region(Var(Symbol::ARG_5)))],
+            CalledVia::Space,
+        );
+
+        let tag_name = TagName::Global("Err".into());
+
+        // ok branch
+        let ok = Tag {
+            variant_var: var_store.fresh(),
+            ext_var: var_store.fresh(),
+            name: tag_name.clone(),
+            arguments: vec![(var_store.fresh(), no_region(call_func))],
+        };
+
+        let pattern = Pattern::AppliedTag {
+            whole_var: result_var,
+            ext_var: var_store.fresh(),
+            tag_name,
+            arguments: vec![(
+                var_store.fresh(),
+                no_region(Pattern::Identifier(Symbol::ARG_5)),
+            )],
+        };
+
+        let branch = WhenBranch {
+            patterns: vec![no_region(pattern)],
+            value: no_region(ok),
+            guard: None,
+        };
+
+        branches.push(branch);
+    }
+
+    {
+        // err branch
+        let tag_name = TagName::Global("Ok".into());
+
+        let err = Tag {
+            variant_var: var_store.fresh(),
+            ext_var: var_store.fresh(),
+            name: tag_name.clone(),
+            arguments: vec![(var_store.fresh(), no_region(Var(Symbol::ARG_4)))],
+        };
+
+        let pattern = Pattern::AppliedTag {
+            whole_var: result_var,
+            ext_var: var_store.fresh(),
+            tag_name,
+            arguments: vec![(
+                var_store.fresh(),
+                no_region(Pattern::Identifier(Symbol::ARG_4)),
+            )],
+        };
+
+        let branch = WhenBranch {
+            patterns: vec![no_region(pattern)],
+            value: no_region(err),
+            guard: None,
+        };
+
+        branches.push(branch);
+    }
+
+    let body = When {
+        cond_var: result_var,
+        expr_var: ret_var,
+        region: Region::zero(),
+        loc_cond: Box::new(no_region(Var(Symbol::ARG_1))),
+        branches,
+    };
+
+    defn(
+        symbol,
+        vec![(result_var, Symbol::ARG_1), (func_var, Symbol::ARG_2)],
+        var_store,
+        body,
+        ret_var,
+    )
+}
+
+fn result_with_default(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let ret_var = var_store.fresh();
+    let result_var = var_store.fresh();
+
+    let mut branches = vec![];
+
+    {
+        // ok branch
+        let tag_name = TagName::Global("Ok".into());
+
+        let pattern = Pattern::AppliedTag {
+            whole_var: result_var,
+            ext_var: var_store.fresh(),
+            tag_name,
+            arguments: vec![(ret_var, no_region(Pattern::Identifier(Symbol::ARG_3)))],
+        };
+
+        let branch = WhenBranch {
+            patterns: vec![no_region(pattern)],
+            value: no_region(Var(Symbol::ARG_3)),
+            guard: None,
+        };
+
+        branches.push(branch);
+    }
+
+    {
+        // err branch
+        let tag_name = TagName::Global("Err".into());
+
+        let pattern = Pattern::AppliedTag {
+            whole_var: result_var,
+            ext_var: var_store.fresh(),
+            tag_name,
+            arguments: vec![(var_store.fresh(), no_region(Pattern::Underscore))],
+        };
+
+        let branch = WhenBranch {
+            patterns: vec![no_region(pattern)],
+            value: no_region(Var(Symbol::ARG_2)),
+            guard: None,
+        };
+
+        branches.push(branch);
+    }
+
+    let body = When {
+        cond_var: result_var,
+        expr_var: ret_var,
+        region: Region::zero(),
+        loc_cond: Box::new(no_region(Var(Symbol::ARG_1))),
+        branches,
+    };
+
+    defn(
+        symbol,
+        vec![(result_var, Symbol::ARG_1), (ret_var, Symbol::ARG_2)],
+        var_store,
+        body,
+        ret_var,
+    )
+}
+
 #[inline(always)]
 fn no_region<T>(value: T) -> Located<T> {
     Located {
diff --git a/compiler/gen/src/llvm/bitcode.rs b/compiler/gen/src/llvm/bitcode.rs
index bfd6557365..bd494641d0 100644
--- a/compiler/gen/src/llvm/bitcode.rs
+++ b/compiler/gen/src/llvm/bitcode.rs
@@ -1,21 +1,376 @@
-use inkwell::types::BasicTypeEnum;
-use roc_module::low_level::LowLevel;
+use crate::debug_info_init;
+use crate::llvm::build::{set_name, Env, FAST_CALL_CONV};
+use crate::llvm::convert::basic_type_from_layout;
+use crate::llvm::refcounting::{decrement_refcount_layout, increment_refcount_layout, Mode};
+use inkwell::attributes::{Attribute, AttributeLoc};
+/// Helpers for interacting with the zig that generates bitcode
+use inkwell::types::{BasicType, BasicTypeEnum};
+use inkwell::values::{BasicValueEnum, CallSiteValue, FunctionValue, InstructionValue};
+use inkwell::AddressSpace;
+use roc_module::symbol::Symbol;
+use roc_mono::layout::{Layout, LayoutIds};
 
 pub fn call_bitcode_fn<'a, 'ctx, 'env>(
-    op: LowLevel,
     env: &Env<'a, 'ctx, 'env>,
     args: &[BasicValueEnum<'ctx>],
     fn_name: &str,
 ) -> BasicValueEnum<'ctx> {
+    call_bitcode_fn_help(env, args, fn_name)
+        .try_as_basic_value()
+        .left()
+        .unwrap_or_else(|| {
+            panic!(
+                "LLVM error: Did not get return value from bitcode function {:?}",
+                fn_name
+            )
+        })
+}
+
+pub fn call_void_bitcode_fn<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    args: &[BasicValueEnum<'ctx>],
+    fn_name: &str,
+) -> InstructionValue<'ctx> {
+    call_bitcode_fn_help(env, args, fn_name)
+        .try_as_basic_value()
+        .right()
+        .unwrap_or_else(|| panic!("LLVM error: Tried to call void bitcode function, but got return value from bitcode function, {:?}", fn_name))
+}
+
+fn call_bitcode_fn_help<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    args: &[BasicValueEnum<'ctx>],
+    fn_name: &str,
+) -> CallSiteValue<'ctx> {
     let fn_val = env
-                .module
-                .get_function(fn_name)
-                .unwrap_or_else(|| panic!("Unrecognized builtin function: {:?} - if you're working on the Roc compiler, do you need to rebuild the bitcode? See compiler/builtins/bitcode/README.md", fn_name));
+        .module
+        .get_function(fn_name)
+        .unwrap_or_else(|| panic!("Unrecognized builtin function: {:?} - if you're working on the Roc compiler, do you need to rebuild the bitcode? See compiler/builtins/bitcode/README.md", fn_name));
+
     let call = env.builder.build_call(fn_val, args, "call_builtin");
 
     call.set_call_convention(fn_val.get_call_conventions());
-
-    call.try_as_basic_value()
-        .left()
-        .unwrap_or_else(|| panic!("LLVM error: Invalid call for low-level op {:?}", op))
+    call
+}
+
+const ARGUMENT_SYMBOLS: [Symbol; 8] = [
+    Symbol::ARG_1,
+    Symbol::ARG_2,
+    Symbol::ARG_3,
+    Symbol::ARG_4,
+    Symbol::ARG_5,
+    Symbol::ARG_6,
+    Symbol::ARG_7,
+    Symbol::ARG_8,
+];
+
+pub fn build_transform_caller<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    function_layout: &Layout<'a>,
+    argument_layouts: &[Layout<'a>],
+) -> FunctionValue<'ctx> {
+    let symbol = Symbol::ZIG_FUNCTION_CALLER;
+    let fn_name = layout_ids
+        .get(symbol, &function_layout)
+        .to_symbol_string(symbol, &env.interns);
+
+    match env.module.get_function(fn_name.as_str()) {
+        Some(function_value) => function_value,
+        None => build_transform_caller_help(env, function_layout, argument_layouts, &fn_name),
+    }
+}
+
+fn build_transform_caller_help<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    function_layout: &Layout<'a>,
+    argument_layouts: &[Layout<'a>],
+    fn_name: &str,
+) -> FunctionValue<'ctx> {
+    debug_assert!(argument_layouts.len() <= 7);
+
+    let block = env.builder.get_insert_block().expect("to be in a function");
+    let di_location = env.builder.get_current_debug_location().unwrap();
+
+    let arg_type = env.context.i8_type().ptr_type(AddressSpace::Generic);
+
+    let function_value = crate::llvm::refcounting::build_header_help(
+        env,
+        &fn_name,
+        env.context.void_type().into(),
+        &(bumpalo::vec![ in env.arena; BasicTypeEnum::PointerType(arg_type); argument_layouts.len() + 2 ]),
+    );
+
+    let kind_id = Attribute::get_named_enum_kind_id("alwaysinline");
+    debug_assert!(kind_id > 0);
+    let attr = env.context.create_enum_attribute(kind_id, 1);
+    function_value.add_attribute(AttributeLoc::Function, attr);
+
+    let entry = env.context.append_basic_block(function_value, "entry");
+    env.builder.position_at_end(entry);
+
+    debug_info_init!(env, function_value);
+
+    let mut it = function_value.get_param_iter();
+    let closure_ptr = it.next().unwrap().into_pointer_value();
+    set_name(closure_ptr.into(), Symbol::ARG_1.ident_string(&env.interns));
+
+    let arguments =
+        bumpalo::collections::Vec::from_iter_in(it.take(argument_layouts.len()), env.arena);
+
+    for (argument, name) in arguments.iter().zip(ARGUMENT_SYMBOLS[1..].iter()) {
+        set_name(*argument, name.ident_string(&env.interns));
+    }
+
+    let closure_type =
+        basic_type_from_layout(env.arena, env.context, function_layout, env.ptr_bytes)
+            .ptr_type(AddressSpace::Generic);
+
+    let mut arguments_cast =
+        bumpalo::collections::Vec::with_capacity_in(arguments.len(), env.arena);
+
+    for (argument_ptr, layout) in arguments.iter().zip(argument_layouts) {
+        let basic_type = basic_type_from_layout(env.arena, env.context, layout, env.ptr_bytes)
+            .ptr_type(AddressSpace::Generic);
+
+        let argument_cast = env
+            .builder
+            .build_bitcast(*argument_ptr, basic_type, "load_opaque")
+            .into_pointer_value();
+
+        let argument = env.builder.build_load(argument_cast, "load_opaque");
+
+        arguments_cast.push(argument);
+    }
+
+    let closure_cast = env
+        .builder
+        .build_bitcast(closure_ptr, closure_type, "load_opaque")
+        .into_pointer_value();
+
+    let fpointer = env.builder.build_load(closure_cast, "load_opaque");
+
+    let call = match function_layout {
+        Layout::FunctionPointer(_, _) => env.builder.build_call(
+            fpointer.into_pointer_value(),
+            arguments_cast.as_slice(),
+            "tmp",
+        ),
+        Layout::Closure(_, _, _) | Layout::Struct(_) => {
+            let pair = fpointer.into_struct_value();
+
+            let fpointer = env
+                .builder
+                .build_extract_value(pair, 0, "get_fpointer")
+                .unwrap();
+
+            let closure_data = env
+                .builder
+                .build_extract_value(pair, 1, "get_closure_data")
+                .unwrap();
+
+            arguments_cast.push(closure_data);
+            env.builder.build_call(
+                fpointer.into_pointer_value(),
+                arguments_cast.as_slice(),
+                "tmp",
+            )
+        }
+        _ => unreachable!("layout is not callable {:?}", function_layout),
+    };
+    call.set_call_convention(FAST_CALL_CONV);
+
+    let result = call
+        .try_as_basic_value()
+        .left()
+        .unwrap_or_else(|| panic!("LLVM error: Invalid call by pointer."));
+
+    let result_u8_ptr = function_value
+        .get_nth_param(argument_layouts.len() as u32 + 1)
+        .unwrap();
+    let result_ptr = env
+        .builder
+        .build_bitcast(
+            result_u8_ptr,
+            result.get_type().ptr_type(AddressSpace::Generic),
+            "write_result",
+        )
+        .into_pointer_value();
+
+    env.builder.build_store(result_ptr, result);
+    env.builder.build_return(None);
+
+    env.builder.position_at_end(block);
+    env.builder
+        .set_current_debug_location(env.context, di_location);
+
+    function_value
+}
+
+pub fn build_inc_wrapper<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    layout: &Layout<'a>,
+) -> FunctionValue<'ctx> {
+    build_rc_wrapper(env, layout_ids, layout, Mode::Inc(1))
+}
+
+pub fn build_dec_wrapper<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    layout: &Layout<'a>,
+) -> FunctionValue<'ctx> {
+    build_rc_wrapper(env, layout_ids, layout, Mode::Dec)
+}
+
+pub fn build_rc_wrapper<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    layout: &Layout<'a>,
+    rc_operation: Mode,
+) -> FunctionValue<'ctx> {
+    let block = env.builder.get_insert_block().expect("to be in a function");
+    let di_location = env.builder.get_current_debug_location().unwrap();
+
+    let symbol = Symbol::GENERIC_RC_REF;
+    let fn_name = layout_ids
+        .get(symbol, &layout)
+        .to_symbol_string(symbol, &env.interns);
+
+    let fn_name = match rc_operation {
+        Mode::Inc(n) => format!("{}_inc_{}", fn_name, n),
+        Mode::Dec => format!("{}_dec", fn_name),
+    };
+
+    let function_value = match env.module.get_function(fn_name.as_str()) {
+        Some(function_value) => function_value,
+        None => {
+            let arg_type = env.context.i8_type().ptr_type(AddressSpace::Generic);
+
+            let function_value = crate::llvm::refcounting::build_header_help(
+                env,
+                &fn_name,
+                env.context.void_type().into(),
+                &[arg_type.into()],
+            );
+
+            let kind_id = Attribute::get_named_enum_kind_id("alwaysinline");
+            debug_assert!(kind_id > 0);
+            let attr = env.context.create_enum_attribute(kind_id, 1);
+            function_value.add_attribute(AttributeLoc::Function, attr);
+
+            let entry = env.context.append_basic_block(function_value, "entry");
+            env.builder.position_at_end(entry);
+
+            debug_info_init!(env, function_value);
+
+            let mut it = function_value.get_param_iter();
+            let value_ptr = it.next().unwrap().into_pointer_value();
+
+            set_name(value_ptr.into(), Symbol::ARG_1.ident_string(&env.interns));
+
+            let value_type = basic_type_from_layout(env.arena, env.context, layout, env.ptr_bytes)
+                .ptr_type(AddressSpace::Generic);
+
+            let value_cast = env
+                .builder
+                .build_bitcast(value_ptr, value_type, "load_opaque")
+                .into_pointer_value();
+
+            let value = env.builder.build_load(value_cast, "load_opaque");
+
+            match rc_operation {
+                Mode::Inc(n) => {
+                    increment_refcount_layout(env, function_value, layout_ids, n, value, layout);
+                }
+                Mode::Dec => {
+                    decrement_refcount_layout(env, function_value, layout_ids, value, layout);
+                }
+            }
+
+            env.builder.build_return(None);
+
+            function_value
+        }
+    };
+
+    env.builder.position_at_end(block);
+    env.builder
+        .set_current_debug_location(env.context, di_location);
+
+    function_value
+}
+
+pub fn build_eq_wrapper<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    layout: &Layout<'a>,
+) -> FunctionValue<'ctx> {
+    let block = env.builder.get_insert_block().expect("to be in a function");
+    let di_location = env.builder.get_current_debug_location().unwrap();
+
+    let symbol = Symbol::GENERIC_EQ_REF;
+    let fn_name = layout_ids
+        .get(symbol, &layout)
+        .to_symbol_string(symbol, &env.interns);
+
+    let function_value = match env.module.get_function(fn_name.as_str()) {
+        Some(function_value) => function_value,
+        None => {
+            let arg_type = env.context.i8_type().ptr_type(AddressSpace::Generic);
+
+            let function_value = crate::llvm::refcounting::build_header_help(
+                env,
+                &fn_name,
+                env.context.bool_type().into(),
+                &[arg_type.into(), arg_type.into()],
+            );
+
+            let kind_id = Attribute::get_named_enum_kind_id("alwaysinline");
+            debug_assert!(kind_id > 0);
+            let attr = env.context.create_enum_attribute(kind_id, 1);
+            function_value.add_attribute(AttributeLoc::Function, attr);
+
+            let entry = env.context.append_basic_block(function_value, "entry");
+            env.builder.position_at_end(entry);
+
+            debug_info_init!(env, function_value);
+
+            let mut it = function_value.get_param_iter();
+            let value_ptr1 = it.next().unwrap().into_pointer_value();
+            let value_ptr2 = it.next().unwrap().into_pointer_value();
+
+            set_name(value_ptr1.into(), Symbol::ARG_1.ident_string(&env.interns));
+            set_name(value_ptr2.into(), Symbol::ARG_2.ident_string(&env.interns));
+
+            let value_type = basic_type_from_layout(env.arena, env.context, layout, env.ptr_bytes)
+                .ptr_type(AddressSpace::Generic);
+
+            let value_cast1 = env
+                .builder
+                .build_bitcast(value_ptr1, value_type, "load_opaque")
+                .into_pointer_value();
+
+            let value_cast2 = env
+                .builder
+                .build_bitcast(value_ptr2, value_type, "load_opaque")
+                .into_pointer_value();
+
+            let value1 = env.builder.build_load(value_cast1, "load_opaque");
+            let value2 = env.builder.build_load(value_cast2, "load_opaque");
+
+            let result =
+                crate::llvm::compare::generic_eq(env, layout_ids, value1, value2, layout, layout);
+
+            env.builder.build_return(Some(&result));
+
+            function_value
+        }
+    };
+
+    env.builder.position_at_end(block);
+    env.builder
+        .set_current_debug_location(env.context, di_location);
+
+    function_value
 }
diff --git a/compiler/gen/src/llvm/build.rs b/compiler/gen/src/llvm/build.rs
index 6ba4296318..df7e65c4a0 100644
--- a/compiler/gen/src/llvm/build.rs
+++ b/compiler/gen/src/llvm/build.rs
@@ -1,14 +1,17 @@
+use crate::llvm::bitcode::call_bitcode_fn;
 use crate::llvm::build_dict::{
-    dict_contains, dict_empty, dict_get, dict_insert, dict_keys, dict_len, dict_remove, dict_values,
+    dict_contains, dict_difference, dict_empty, dict_get, dict_insert, dict_intersection,
+    dict_keys, dict_len, dict_remove, dict_union, dict_values, dict_walk, set_from_list,
 };
 use crate::llvm::build_hash::generic_hash;
 use crate::llvm::build_list::{
     allocate_list, empty_list, empty_polymorphic_list, list_append, list_concat, list_contains,
-    list_get_unsafe, list_join, list_keep_if, list_len, list_map, list_prepend, list_repeat,
-    list_reverse, list_set, list_single, list_sum, list_walk, list_walk_backwards,
+    list_get_unsafe, list_join, list_keep_errs, list_keep_if, list_keep_oks, list_len, list_map,
+    list_map_with_index, list_prepend, list_repeat, list_reverse, list_set, list_single, list_sum,
+    list_walk, list_walk_backwards,
 };
 use crate::llvm::build_str::{
-    str_concat, str_count_graphemes, str_ends_with, str_from_int, str_join_with,
+    str_concat, str_count_graphemes, str_ends_with, str_from_float, str_from_int, str_join_with,
     str_number_of_bytes, str_split, str_starts_with, CHAR_LAYOUT,
 };
 use crate::llvm::compare::{generic_eq, generic_neq};
@@ -35,8 +38,8 @@ use inkwell::passes::{PassManager, PassManagerBuilder};
 use inkwell::types::{BasicTypeEnum, FunctionType, IntType, StructType};
 use inkwell::values::BasicValueEnum::{self, *};
 use inkwell::values::{
-    BasicValue, CallSiteValue, FloatValue, FunctionValue, InstructionOpcode, InstructionValue,
-    IntValue, PointerValue, StructValue,
+    BasicValue, CallSiteValue, FloatValue, FunctionValue, InstructionOpcode, IntValue,
+    PointerValue, StructValue,
 };
 use inkwell::OptimizationLevel;
 use inkwell::{AddressSpace, IntPredicate};
@@ -753,8 +756,8 @@ pub fn build_exp_call<'a, 'ctx, 'env>(
                 }
                 non_ptr => {
                     panic!(
-                        "Tried to call by pointer, but encountered a non-pointer: {:?}",
-                        non_ptr
+                        "Tried to call by pointer, but encountered a non-pointer: {:?} {:?} {:?}",
+                        name, non_ptr, full_layout
                     );
                 }
             };
@@ -1850,6 +1853,7 @@ fn invoke_roc_function<'a, 'ctx, 'env>(
     layout: Layout<'a>,
     function_value: Either<FunctionValue<'ctx>, PointerValue<'ctx>>,
     arguments: &[Symbol],
+    closure_argument: Option<BasicValueEnum<'ctx>>,
     pass: &'a roc_mono::ir::Stmt<'a>,
     fail: &'a roc_mono::ir::Stmt<'a>,
 ) -> BasicValueEnum<'ctx> {
@@ -1860,6 +1864,7 @@ fn invoke_roc_function<'a, 'ctx, 'env>(
     for arg in arguments.iter() {
         arg_vals.push(load_symbol(scope, arg));
     }
+    arg_vals.extend(closure_argument);
 
     let pass_block = context.append_basic_block(parent, "invoke_pass");
     let fail_block = context.append_basic_block(parent, "invoke_fail");
@@ -2019,6 +2024,7 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
                     layout.clone(),
                     function_value.into(),
                     call.arguments,
+                    None,
                     pass,
                     fail,
                 )
@@ -2026,28 +2032,57 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
             CallType::ByPointer { name, .. } => {
                 let sub_expr = load_symbol(scope, &name);
 
-                let function_ptr = match sub_expr {
-                    BasicValueEnum::PointerValue(ptr) => ptr,
+                match sub_expr {
+                    BasicValueEnum::PointerValue(function_ptr) => {
+                        // basic call by pointer
+                        invoke_roc_function(
+                            env,
+                            layout_ids,
+                            scope,
+                            parent,
+                            *symbol,
+                            layout.clone(),
+                            function_ptr.into(),
+                            call.arguments,
+                            None,
+                            pass,
+                            fail,
+                        )
+                    }
+                    BasicValueEnum::StructValue(ptr_and_data) => {
+                        // this is a closure
+                        let builder = env.builder;
+
+                        let function_ptr = builder
+                            .build_extract_value(ptr_and_data, 0, "function_ptr")
+                            .unwrap()
+                            .into_pointer_value();
+
+                        let closure_data = builder
+                            .build_extract_value(ptr_and_data, 1, "closure_data")
+                            .unwrap();
+
+                        invoke_roc_function(
+                            env,
+                            layout_ids,
+                            scope,
+                            parent,
+                            *symbol,
+                            layout.clone(),
+                            function_ptr.into(),
+                            call.arguments,
+                            Some(closure_data),
+                            pass,
+                            fail,
+                        )
+                    }
                     non_ptr => {
                         panic!(
                             "Tried to call by pointer, but encountered a non-pointer: {:?}",
                             non_ptr
                         );
                     }
-                };
-
-                invoke_roc_function(
-                    env,
-                    layout_ids,
-                    scope,
-                    parent,
-                    *symbol,
-                    layout.clone(),
-                    function_ptr.into(),
-                    call.arguments,
-                    pass,
-                    fail,
-                )
+                }
             }
             CallType::Foreign {
                 ref foreign_symbol,
@@ -2203,9 +2238,14 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
                     let (value, layout) = load_symbol_and_layout(scope, symbol);
 
                     if layout.is_refcounted() {
-                        let value_ptr = value.into_pointer_value();
-                        let refcount_ptr = PointerToRefcount::from_ptr_to_data(env, value_ptr);
-                        refcount_ptr.decrement(env, layout);
+                        if value.is_pointer_value() {
+                            // BasicValueEnum::PointerValue(value_ptr) => {
+                            let value_ptr = value.into_pointer_value();
+                            let refcount_ptr = PointerToRefcount::from_ptr_to_data(env, value_ptr);
+                            refcount_ptr.decrement(env, layout);
+                        } else {
+                            eprint!("we're likely leaking memory; see issue #985 for details");
+                        }
                     }
 
                     build_exp_stmt(env, layout_ids, scope, parent, cont)
@@ -3497,6 +3537,12 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             str_from_int(env, scope, args[0])
         }
+        StrFromFloat => {
+            // Str.fromFloat : Float * -> Str
+            debug_assert_eq!(args.len(), 1);
+
+            str_from_float(env, scope, args[0])
+        }
         StrSplit => {
             // Str.split : Str, Str -> List Str
             debug_assert_eq!(args.len(), 2);
@@ -3582,18 +3628,29 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             let (func, func_layout) = load_symbol_and_layout(scope, &args[1]);
 
-            let inplace = get_inplace_from_layout(layout);
+            match list_layout {
+                Layout::Builtin(Builtin::EmptyList) => empty_list(env),
+                Layout::Builtin(Builtin::List(_, element_layout)) => {
+                    list_map(env, layout_ids, func, func_layout, list, element_layout)
+                }
+                _ => unreachable!("invalid list layout"),
+            }
+        }
+        ListMapWithIndex => {
+            // List.map : List before, (before -> after) -> List after
+            debug_assert_eq!(args.len(), 2);
 
-            list_map(
-                env,
-                layout_ids,
-                inplace,
-                parent,
-                func,
-                func_layout,
-                list,
-                list_layout,
-            )
+            let (list, list_layout) = load_symbol_and_layout(scope, &args[0]);
+
+            let (func, func_layout) = load_symbol_and_layout(scope, &args[1]);
+
+            match list_layout {
+                Layout::Builtin(Builtin::EmptyList) => empty_list(env),
+                Layout::Builtin(Builtin::List(_, element_layout)) => {
+                    list_map_with_index(env, layout_ids, func, func_layout, list, element_layout)
+                }
+                _ => unreachable!("invalid list layout"),
+            }
         }
         ListKeepIf => {
             // List.keepIf : List elem, (elem -> Bool) -> List elem
@@ -3603,36 +3660,79 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             let (func, func_layout) = load_symbol_and_layout(scope, &args[1]);
 
-            let inplace = get_inplace_from_layout(layout);
+            match list_layout {
+                Layout::Builtin(Builtin::EmptyList) => empty_list(env),
+                Layout::Builtin(Builtin::List(_, element_layout)) => {
+                    list_keep_if(env, layout_ids, func, func_layout, list, element_layout)
+                }
+                _ => unreachable!("invalid list layout"),
+            }
+        }
+        ListKeepOks => {
+            // List.keepOks : List before, (before -> Result after *) -> List after
+            debug_assert_eq!(args.len(), 2);
 
-            list_keep_if(
-                env,
-                layout_ids,
-                inplace,
-                parent,
-                func,
-                func_layout,
-                list,
-                list_layout,
-            )
+            let (list, list_layout) = load_symbol_and_layout(scope, &args[0]);
+
+            let (func, func_layout) = load_symbol_and_layout(scope, &args[1]);
+
+            match (list_layout, layout) {
+                (_, Layout::Builtin(Builtin::EmptyList))
+                | (Layout::Builtin(Builtin::EmptyList), _) => empty_list(env),
+                (
+                    Layout::Builtin(Builtin::List(_, before_layout)),
+                    Layout::Builtin(Builtin::List(_, after_layout)),
+                ) => list_keep_oks(
+                    env,
+                    layout_ids,
+                    func,
+                    func_layout,
+                    list,
+                    before_layout,
+                    after_layout,
+                ),
+                (other1, other2) => {
+                    unreachable!("invalid list layouts:\n{:?}\n{:?}", other1, other2)
+                }
+            }
+        }
+        ListKeepErrs => {
+            // List.keepErrs : List before, (before -> Result * after) -> List after
+            debug_assert_eq!(args.len(), 2);
+
+            let (list, list_layout) = load_symbol_and_layout(scope, &args[0]);
+
+            let (func, func_layout) = load_symbol_and_layout(scope, &args[1]);
+
+            match (list_layout, layout) {
+                (_, Layout::Builtin(Builtin::EmptyList))
+                | (Layout::Builtin(Builtin::EmptyList), _) => empty_list(env),
+                (
+                    Layout::Builtin(Builtin::List(_, before_layout)),
+                    Layout::Builtin(Builtin::List(_, after_layout)),
+                ) => list_keep_errs(
+                    env,
+                    layout_ids,
+                    func,
+                    func_layout,
+                    list,
+                    before_layout,
+                    after_layout,
+                ),
+                (other1, other2) => {
+                    unreachable!("invalid list layouts:\n{:?}\n{:?}", other1, other2)
+                }
+            }
         }
         ListContains => {
             // List.contains : List elem, elem -> Bool
             debug_assert_eq!(args.len(), 2);
 
-            let (list, list_layout) = load_symbol_and_layout(scope, &args[0]);
+            let list = load_symbol(scope, &args[0]);
 
             let (elem, elem_layout) = load_symbol_and_layout(scope, &args[1]);
 
-            list_contains(
-                env,
-                layout_ids,
-                parent,
-                elem,
-                elem_layout,
-                list,
-                list_layout,
-            )
+            list_contains(env, layout_ids, elem, elem_layout, list)
         }
         ListWalk => {
             debug_assert_eq!(args.len(), 3);
@@ -3643,16 +3743,21 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             let (default, default_layout) = load_symbol_and_layout(scope, &args[2]);
 
-            list_walk(
-                env,
-                parent,
-                list,
-                list_layout,
-                func,
-                func_layout,
-                default,
-                default_layout,
-            )
+            match list_layout {
+                Layout::Builtin(Builtin::EmptyList) => default,
+                Layout::Builtin(Builtin::List(_, element_layout)) => list_walk(
+                    env,
+                    layout_ids,
+                    parent,
+                    list,
+                    element_layout,
+                    func,
+                    func_layout,
+                    default,
+                    default_layout,
+                ),
+                _ => unreachable!("invalid list layout"),
+            }
         }
         ListWalkBackwards => {
             // List.walkBackwards : List elem, (elem -> accum -> accum), accum -> accum
@@ -3664,16 +3769,21 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             let (default, default_layout) = load_symbol_and_layout(scope, &args[2]);
 
-            list_walk_backwards(
-                env,
-                parent,
-                list,
-                list_layout,
-                func,
-                func_layout,
-                default,
-                default_layout,
-            )
+            match list_layout {
+                Layout::Builtin(Builtin::EmptyList) => default,
+                Layout::Builtin(Builtin::List(_, element_layout)) => list_walk_backwards(
+                    env,
+                    layout_ids,
+                    parent,
+                    list,
+                    element_layout,
+                    func,
+                    func_layout,
+                    default,
+                    default_layout,
+                ),
+                _ => unreachable!("invalid list layout"),
+            }
         }
         ListSum => {
             debug_assert_eq!(args.len(), 1);
@@ -4047,7 +4157,7 @@ fn run_low_level<'a, 'ctx, 'env>(
             match dict_layout {
                 Layout::Builtin(Builtin::EmptyDict) => {
                     // no elements, so `key` is not in here
-                    panic!("key type unknown")
+                    empty_list(env)
                 }
                 Layout::Builtin(Builtin::Dict(key_layout, value_layout)) => {
                     dict_keys(env, layout_ids, dict, key_layout, value_layout)
@@ -4063,7 +4173,7 @@ fn run_low_level<'a, 'ctx, 'env>(
             match dict_layout {
                 Layout::Builtin(Builtin::EmptyDict) => {
                     // no elements, so `key` is not in here
-                    panic!("key type unknown")
+                    empty_list(env)
                 }
                 Layout::Builtin(Builtin::Dict(key_layout, value_layout)) => {
                     dict_values(env, layout_ids, dict, key_layout, value_layout)
@@ -4071,6 +4181,96 @@ fn run_low_level<'a, 'ctx, 'env>(
                 _ => unreachable!("invalid dict layout"),
             }
         }
+        DictUnion => {
+            debug_assert_eq!(args.len(), 2);
+
+            let (dict1, dict_layout) = load_symbol_and_layout(scope, &args[0]);
+            let (dict2, _) = load_symbol_and_layout(scope, &args[1]);
+
+            match dict_layout {
+                Layout::Builtin(Builtin::EmptyDict) => {
+                    // no elements, so `key` is not in here
+                    panic!("key type unknown")
+                }
+                Layout::Builtin(Builtin::Dict(key_layout, value_layout)) => {
+                    dict_union(env, layout_ids, dict1, dict2, key_layout, value_layout)
+                }
+                _ => unreachable!("invalid dict layout"),
+            }
+        }
+        DictDifference => {
+            debug_assert_eq!(args.len(), 2);
+
+            let (dict1, dict_layout) = load_symbol_and_layout(scope, &args[0]);
+            let (dict2, _) = load_symbol_and_layout(scope, &args[1]);
+
+            match dict_layout {
+                Layout::Builtin(Builtin::EmptyDict) => {
+                    // no elements, so `key` is not in here
+                    panic!("key type unknown")
+                }
+                Layout::Builtin(Builtin::Dict(key_layout, value_layout)) => {
+                    dict_difference(env, layout_ids, dict1, dict2, key_layout, value_layout)
+                }
+                _ => unreachable!("invalid dict layout"),
+            }
+        }
+        DictIntersection => {
+            debug_assert_eq!(args.len(), 2);
+
+            let (dict1, dict_layout) = load_symbol_and_layout(scope, &args[0]);
+            let (dict2, _) = load_symbol_and_layout(scope, &args[1]);
+
+            match dict_layout {
+                Layout::Builtin(Builtin::EmptyDict) => {
+                    // no elements, so `key` is not in here
+                    panic!("key type unknown")
+                }
+                Layout::Builtin(Builtin::Dict(key_layout, value_layout)) => {
+                    dict_intersection(env, layout_ids, dict1, dict2, key_layout, value_layout)
+                }
+                _ => unreachable!("invalid dict layout"),
+            }
+        }
+        DictWalk => {
+            debug_assert_eq!(args.len(), 3);
+
+            let (dict, dict_layout) = load_symbol_and_layout(scope, &args[0]);
+            let (stepper, stepper_layout) = load_symbol_and_layout(scope, &args[1]);
+            let (accum, accum_layout) = load_symbol_and_layout(scope, &args[2]);
+
+            match dict_layout {
+                Layout::Builtin(Builtin::EmptyDict) => {
+                    // no elements, so `key` is not in here
+                    panic!("key type unknown")
+                }
+                Layout::Builtin(Builtin::Dict(key_layout, value_layout)) => dict_walk(
+                    env,
+                    layout_ids,
+                    dict,
+                    stepper,
+                    accum,
+                    stepper_layout,
+                    key_layout,
+                    value_layout,
+                    accum_layout,
+                ),
+                _ => unreachable!("invalid dict layout"),
+            }
+        }
+        SetFromList => {
+            debug_assert_eq!(args.len(), 1);
+
+            let (list, list_layout) = load_symbol_and_layout(scope, &args[0]);
+
+            match list_layout {
+                Layout::Builtin(Builtin::EmptyList) => dict_empty(env, scope),
+                Layout::Builtin(Builtin::List(_, key_layout)) => {
+                    set_from_list(env, layout_ids, list, key_layout)
+                }
+                _ => unreachable!("invalid dict layout"),
+            }
+        }
     }
 }
 
@@ -4393,49 +4593,6 @@ fn build_int_binop<'a, 'ctx, 'env>(
     }
 }
 
-pub fn call_bitcode_fn<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    args: &[BasicValueEnum<'ctx>],
-    fn_name: &str,
-) -> BasicValueEnum<'ctx> {
-    call_bitcode_fn_help(env, args, fn_name)
-        .try_as_basic_value()
-        .left()
-        .unwrap_or_else(|| {
-            panic!(
-                "LLVM error: Did not get return value from bitcode function {:?}",
-                fn_name
-            )
-        })
-}
-
-pub fn call_void_bitcode_fn<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    args: &[BasicValueEnum<'ctx>],
-    fn_name: &str,
-) -> InstructionValue<'ctx> {
-    call_bitcode_fn_help(env, args, fn_name)
-        .try_as_basic_value()
-        .right()
-        .unwrap_or_else(|| panic!("LLVM error: Tried to call void bitcode function, but got return value from bitcode function, {:?}", fn_name))
-}
-
-fn call_bitcode_fn_help<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    args: &[BasicValueEnum<'ctx>],
-    fn_name: &str,
-) -> CallSiteValue<'ctx> {
-    let fn_val = env
-        .module
-        .get_function(fn_name)
-        .unwrap_or_else(|| panic!("Unrecognized builtin function: {:?} - if you're working on the Roc compiler, do you need to rebuild the bitcode? See compiler/builtins/bitcode/README.md", fn_name));
-
-    let call = env.builder.build_call(fn_val, args, "call_builtin");
-
-    call.set_call_convention(fn_val.get_call_conventions());
-    call
-}
-
 pub fn build_num_binop<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     parent: FunctionValue<'ctx>,
diff --git a/compiler/gen/src/llvm/build_dict.rs b/compiler/gen/src/llvm/build_dict.rs
index 3b8cd5feed..8aa6f17887 100644
--- a/compiler/gen/src/llvm/build_dict.rs
+++ b/compiler/gen/src/llvm/build_dict.rs
@@ -1,10 +1,13 @@
 use crate::debug_info_init;
+use crate::llvm::bitcode::{
+    build_dec_wrapper, build_eq_wrapper, build_inc_wrapper, build_transform_caller,
+    call_bitcode_fn, call_void_bitcode_fn,
+};
 use crate::llvm::build::{
-    call_bitcode_fn, call_void_bitcode_fn, complex_bitcast, load_symbol, load_symbol_and_layout,
-    set_name, Env, Scope,
+    complex_bitcast, load_symbol, load_symbol_and_layout, set_name, Env, Scope,
 };
 use crate::llvm::convert::{self, as_const_zero, basic_type_from_layout, collection};
-use crate::llvm::refcounting::{decrement_refcount_layout, increment_refcount_layout, Mode};
+use crate::llvm::refcounting::Mode;
 use inkwell::attributes::{Attribute, AttributeLoc};
 use inkwell::types::BasicType;
 use inkwell::values::{BasicValueEnum, FunctionValue, StructValue};
@@ -128,8 +131,8 @@ pub fn dict_insert<'a, 'ctx, 'env>(
     let hash_fn = build_hash_wrapper(env, layout_ids, key_layout);
     let eq_fn = build_eq_wrapper(env, layout_ids, key_layout);
 
-    let dec_key_fn = build_rc_wrapper(env, layout_ids, key_layout, Mode::Dec);
-    let dec_value_fn = build_rc_wrapper(env, layout_ids, value_layout, Mode::Dec);
+    let dec_key_fn = build_dec_wrapper(env, layout_ids, key_layout);
+    let dec_value_fn = build_dec_wrapper(env, layout_ids, value_layout);
 
     call_void_bitcode_fn(
         env,
@@ -198,8 +201,8 @@ pub fn dict_remove<'a, 'ctx, 'env>(
     let hash_fn = build_hash_wrapper(env, layout_ids, key_layout);
     let eq_fn = build_eq_wrapper(env, layout_ids, key_layout);
 
-    let dec_key_fn = build_rc_wrapper(env, layout_ids, key_layout, Mode::Dec);
-    let dec_value_fn = build_rc_wrapper(env, layout_ids, value_layout, Mode::Dec);
+    let dec_key_fn = build_dec_wrapper(env, layout_ids, key_layout);
+    let dec_value_fn = build_dec_wrapper(env, layout_ids, value_layout);
 
     call_void_bitcode_fn(
         env,
@@ -315,7 +318,7 @@ pub fn dict_get<'a, 'ctx, 'env>(
     let hash_fn = build_hash_wrapper(env, layout_ids, key_layout);
     let eq_fn = build_eq_wrapper(env, layout_ids, key_layout);
 
-    let inc_value_fn = build_rc_wrapper(env, layout_ids, value_layout, Mode::Inc(1));
+    let inc_value_fn = build_inc_wrapper(env, layout_ids, value_layout);
 
     // { flag: bool, value: *const u8 }
     let result = call_bitcode_fn(
@@ -422,6 +425,7 @@ pub fn dict_elements_rc<'a, 'ctx, 'env>(
     let alignment = Alignment::from_key_value_layout(key_layout, value_layout, env.ptr_bytes);
     let alignment_iv = env.context.i8_type().const_int(alignment as u64, false);
 
+    use crate::llvm::bitcode::build_rc_wrapper;
     let inc_key_fn = build_rc_wrapper(env, layout_ids, key_layout, rc_operation);
     let inc_value_fn = build_rc_wrapper(env, layout_ids, value_layout, rc_operation);
 
@@ -450,7 +454,7 @@ pub fn dict_keys<'a, 'ctx, 'env>(
     let builder = env.builder;
 
     let zig_dict_type = env.module.get_struct_type("dict.RocDict").unwrap();
-    let zig_list_type = env.module.get_struct_type("dict.RocList").unwrap();
+    let zig_list_type = env.module.get_struct_type("list.RocList").unwrap();
 
     let dict_ptr = builder.build_alloca(zig_dict_type, "dict_ptr");
     env.builder
@@ -467,7 +471,7 @@ pub fn dict_keys<'a, 'ctx, 'env>(
     let alignment = Alignment::from_key_value_layout(key_layout, value_layout, env.ptr_bytes);
     let alignment_iv = env.context.i8_type().const_int(alignment as u64, false);
 
-    let inc_key_fn = build_rc_wrapper(env, layout_ids, key_layout, Mode::Inc(1));
+    let inc_key_fn = build_inc_wrapper(env, layout_ids, key_layout);
 
     let list_ptr = builder.build_alloca(zig_list_type, "list_ptr");
 
@@ -496,6 +500,272 @@ pub fn dict_keys<'a, 'ctx, 'env>(
     env.builder.build_load(list_ptr, "load_keys_list")
 }
 
+#[allow(clippy::too_many_arguments)]
+pub fn dict_union<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    dict1: BasicValueEnum<'ctx>,
+    dict2: BasicValueEnum<'ctx>,
+    key_layout: &Layout<'a>,
+    value_layout: &Layout<'a>,
+) -> BasicValueEnum<'ctx> {
+    let builder = env.builder;
+
+    let zig_dict_type = env.module.get_struct_type("dict.RocDict").unwrap();
+
+    let dict1_ptr = builder.build_alloca(zig_dict_type, "dict_ptr");
+    let dict2_ptr = builder.build_alloca(zig_dict_type, "dict_ptr");
+
+    env.builder.build_store(
+        dict1_ptr,
+        struct_to_zig_dict(env, dict1.into_struct_value()),
+    );
+
+    env.builder.build_store(
+        dict2_ptr,
+        struct_to_zig_dict(env, dict2.into_struct_value()),
+    );
+
+    let key_width = env
+        .ptr_int()
+        .const_int(key_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let value_width = env
+        .ptr_int()
+        .const_int(value_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let alignment = Alignment::from_key_value_layout(key_layout, value_layout, env.ptr_bytes);
+    let alignment_iv = env.context.i8_type().const_int(alignment as u64, false);
+
+    let hash_fn = build_hash_wrapper(env, layout_ids, key_layout);
+    let eq_fn = build_eq_wrapper(env, layout_ids, key_layout);
+
+    let inc_key_fn = build_inc_wrapper(env, layout_ids, key_layout);
+    let inc_value_fn = build_inc_wrapper(env, layout_ids, value_layout);
+
+    let output_ptr = builder.build_alloca(zig_dict_type, "output_ptr");
+
+    call_void_bitcode_fn(
+        env,
+        &[
+            dict1_ptr.into(),
+            dict2_ptr.into(),
+            alignment_iv.into(),
+            key_width.into(),
+            value_width.into(),
+            hash_fn.as_global_value().as_pointer_value().into(),
+            eq_fn.as_global_value().as_pointer_value().into(),
+            inc_key_fn.as_global_value().as_pointer_value().into(),
+            inc_value_fn.as_global_value().as_pointer_value().into(),
+            output_ptr.into(),
+        ],
+        &bitcode::DICT_UNION,
+    );
+
+    let output_ptr = env
+        .builder
+        .build_bitcast(
+            output_ptr,
+            convert::dict(env.context, env.ptr_bytes).ptr_type(AddressSpace::Generic),
+            "to_roc_dict",
+        )
+        .into_pointer_value();
+
+    env.builder.build_load(output_ptr, "load_output_ptr")
+}
+
+#[allow(clippy::too_many_arguments)]
+pub fn dict_difference<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    dict1: BasicValueEnum<'ctx>,
+    dict2: BasicValueEnum<'ctx>,
+    key_layout: &Layout<'a>,
+    value_layout: &Layout<'a>,
+) -> BasicValueEnum<'ctx> {
+    dict_intersect_or_difference(
+        env,
+        layout_ids,
+        dict1,
+        dict2,
+        key_layout,
+        value_layout,
+        &bitcode::DICT_DIFFERENCE,
+    )
+}
+
+#[allow(clippy::too_many_arguments)]
+pub fn dict_intersection<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    dict1: BasicValueEnum<'ctx>,
+    dict2: BasicValueEnum<'ctx>,
+    key_layout: &Layout<'a>,
+    value_layout: &Layout<'a>,
+) -> BasicValueEnum<'ctx> {
+    dict_intersect_or_difference(
+        env,
+        layout_ids,
+        dict1,
+        dict2,
+        key_layout,
+        value_layout,
+        &bitcode::DICT_INTERSECTION,
+    )
+}
+
+#[allow(clippy::too_many_arguments)]
+fn dict_intersect_or_difference<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    dict1: BasicValueEnum<'ctx>,
+    dict2: BasicValueEnum<'ctx>,
+    key_layout: &Layout<'a>,
+    value_layout: &Layout<'a>,
+    op: &str,
+) -> BasicValueEnum<'ctx> {
+    let builder = env.builder;
+
+    let zig_dict_type = env.module.get_struct_type("dict.RocDict").unwrap();
+
+    let dict1_ptr = builder.build_alloca(zig_dict_type, "dict_ptr");
+    let dict2_ptr = builder.build_alloca(zig_dict_type, "dict_ptr");
+
+    env.builder.build_store(
+        dict1_ptr,
+        struct_to_zig_dict(env, dict1.into_struct_value()),
+    );
+
+    env.builder.build_store(
+        dict2_ptr,
+        struct_to_zig_dict(env, dict2.into_struct_value()),
+    );
+
+    let key_width = env
+        .ptr_int()
+        .const_int(key_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let value_width = env
+        .ptr_int()
+        .const_int(value_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let alignment = Alignment::from_key_value_layout(key_layout, value_layout, env.ptr_bytes);
+    let alignment_iv = env.context.i8_type().const_int(alignment as u64, false);
+
+    let hash_fn = build_hash_wrapper(env, layout_ids, key_layout);
+    let eq_fn = build_eq_wrapper(env, layout_ids, key_layout);
+
+    let dec_key_fn = build_dec_wrapper(env, layout_ids, key_layout);
+    let dec_value_fn = build_dec_wrapper(env, layout_ids, value_layout);
+
+    let output_ptr = builder.build_alloca(zig_dict_type, "output_ptr");
+
+    call_void_bitcode_fn(
+        env,
+        &[
+            dict1_ptr.into(),
+            dict2_ptr.into(),
+            alignment_iv.into(),
+            key_width.into(),
+            value_width.into(),
+            hash_fn.as_global_value().as_pointer_value().into(),
+            eq_fn.as_global_value().as_pointer_value().into(),
+            dec_key_fn.as_global_value().as_pointer_value().into(),
+            dec_value_fn.as_global_value().as_pointer_value().into(),
+            output_ptr.into(),
+        ],
+        op,
+    );
+
+    let output_ptr = env
+        .builder
+        .build_bitcast(
+            output_ptr,
+            convert::dict(env.context, env.ptr_bytes).ptr_type(AddressSpace::Generic),
+            "to_roc_dict",
+        )
+        .into_pointer_value();
+
+    env.builder.build_load(output_ptr, "load_output_ptr")
+}
+
+#[allow(clippy::too_many_arguments)]
+pub fn dict_walk<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    dict: BasicValueEnum<'ctx>,
+    stepper: BasicValueEnum<'ctx>,
+    accum: BasicValueEnum<'ctx>,
+    stepper_layout: &Layout<'a>,
+    key_layout: &Layout<'a>,
+    value_layout: &Layout<'a>,
+    accum_layout: &Layout<'a>,
+) -> BasicValueEnum<'ctx> {
+    let builder = env.builder;
+
+    let u8_ptr = env.context.i8_type().ptr_type(AddressSpace::Generic);
+    let zig_dict_type = env.module.get_struct_type("dict.RocDict").unwrap();
+
+    let dict_ptr = builder.build_alloca(zig_dict_type, "dict_ptr");
+    env.builder
+        .build_store(dict_ptr, struct_to_zig_dict(env, dict.into_struct_value()));
+
+    let stepper_ptr = builder.build_alloca(stepper.get_type(), "stepper_ptr");
+    env.builder.build_store(stepper_ptr, stepper);
+
+    let stepper_caller = build_transform_caller(
+        env,
+        layout_ids,
+        stepper_layout,
+        &[
+            key_layout.clone(),
+            value_layout.clone(),
+            accum_layout.clone(),
+        ],
+    )
+    .as_global_value()
+    .as_pointer_value();
+
+    let accum_bt = basic_type_from_layout(env.arena, env.context, accum_layout, env.ptr_bytes);
+    let accum_ptr = builder.build_alloca(accum_bt, "accum_ptr");
+    env.builder.build_store(accum_ptr, accum);
+
+    let key_width = env
+        .ptr_int()
+        .const_int(key_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let value_width = env
+        .ptr_int()
+        .const_int(value_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let accum_width = env
+        .ptr_int()
+        .const_int(accum_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let alignment = Alignment::from_key_value_layout(key_layout, value_layout, env.ptr_bytes);
+    let alignment_iv = env.context.i8_type().const_int(alignment as u64, false);
+
+    let output_ptr = builder.build_alloca(accum_bt, "output_ptr");
+
+    call_void_bitcode_fn(
+        env,
+        &[
+            dict_ptr.into(),
+            env.builder.build_bitcast(stepper_ptr, u8_ptr, "to_opaque"),
+            stepper_caller.into(),
+            env.builder.build_bitcast(accum_ptr, u8_ptr, "to_opaque"),
+            alignment_iv.into(),
+            key_width.into(),
+            value_width.into(),
+            accum_width.into(),
+            env.builder.build_bitcast(output_ptr, u8_ptr, "to_opaque"),
+        ],
+        &bitcode::DICT_WALK,
+    );
+
+    env.builder.build_load(output_ptr, "load_output_ptr")
+}
+
 #[allow(clippy::too_many_arguments)]
 pub fn dict_values<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
@@ -507,7 +777,7 @@ pub fn dict_values<'a, 'ctx, 'env>(
     let builder = env.builder;
 
     let zig_dict_type = env.module.get_struct_type("dict.RocDict").unwrap();
-    let zig_list_type = env.module.get_struct_type("dict.RocList").unwrap();
+    let zig_list_type = env.module.get_struct_type("list.RocList").unwrap();
 
     let dict_ptr = builder.build_alloca(zig_dict_type, "dict_ptr");
     env.builder
@@ -524,7 +794,7 @@ pub fn dict_values<'a, 'ctx, 'env>(
     let alignment = Alignment::from_key_value_layout(key_layout, value_layout, env.ptr_bytes);
     let alignment_iv = env.context.i8_type().const_int(alignment as u64, false);
 
-    let inc_value_fn = build_rc_wrapper(env, layout_ids, value_layout, Mode::Inc(1));
+    let inc_value_fn = build_inc_wrapper(env, layout_ids, value_layout);
 
     let list_ptr = builder.build_alloca(zig_list_type, "list_ptr");
 
@@ -553,6 +823,68 @@ pub fn dict_values<'a, 'ctx, 'env>(
     env.builder.build_load(list_ptr, "load_keys_list")
 }
 
+#[allow(clippy::too_many_arguments)]
+pub fn set_from_list<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    list: BasicValueEnum<'ctx>,
+    key_layout: &Layout<'a>,
+) -> BasicValueEnum<'ctx> {
+    let builder = env.builder;
+
+    let zig_dict_type = env.module.get_struct_type("dict.RocDict").unwrap();
+
+    let list_alloca = builder.build_alloca(list.get_type(), "list_alloca");
+    let list_ptr = env.builder.build_bitcast(
+        list_alloca,
+        env.context.i128_type().ptr_type(AddressSpace::Generic),
+        "to_zig_list",
+    );
+
+    env.builder.build_store(list_alloca, list);
+
+    let key_width = env
+        .ptr_int()
+        .const_int(key_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let value_width = env.ptr_int().const_zero();
+
+    let result_alloca =
+        builder.build_alloca(convert::dict(env.context, env.ptr_bytes), "result_alloca");
+    let result_ptr = builder.build_bitcast(
+        result_alloca,
+        zig_dict_type.ptr_type(AddressSpace::Generic),
+        "to_zig_dict",
+    );
+
+    let alignment =
+        Alignment::from_key_value_layout(key_layout, &Layout::Struct(&[]), env.ptr_bytes);
+    let alignment_iv = env.context.i8_type().const_int(alignment as u64, false);
+
+    let hash_fn = build_hash_wrapper(env, layout_ids, key_layout);
+    let eq_fn = build_eq_wrapper(env, layout_ids, key_layout);
+
+    let dec_key_fn = build_dec_wrapper(env, layout_ids, key_layout);
+
+    call_void_bitcode_fn(
+        env,
+        &[
+            env.builder
+                .build_load(list_ptr.into_pointer_value(), "as_i128"),
+            alignment_iv.into(),
+            key_width.into(),
+            value_width.into(),
+            hash_fn.as_global_value().as_pointer_value().into(),
+            eq_fn.as_global_value().as_pointer_value().into(),
+            dec_key_fn.as_global_value().as_pointer_value().into(),
+            result_ptr,
+        ],
+        &bitcode::SET_FROM_LIST,
+    );
+
+    env.builder.build_load(result_alloca, "load_result")
+}
+
 fn build_hash_wrapper<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
@@ -622,158 +954,6 @@ fn build_hash_wrapper<'a, 'ctx, 'env>(
     function_value
 }
 
-fn build_eq_wrapper<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    layout_ids: &mut LayoutIds<'a>,
-    layout: &Layout<'a>,
-) -> FunctionValue<'ctx> {
-    let block = env.builder.get_insert_block().expect("to be in a function");
-    let di_location = env.builder.get_current_debug_location().unwrap();
-
-    let symbol = Symbol::GENERIC_EQ_REF;
-    let fn_name = layout_ids
-        .get(symbol, &layout)
-        .to_symbol_string(symbol, &env.interns);
-
-    let function_value = match env.module.get_function(fn_name.as_str()) {
-        Some(function_value) => function_value,
-        None => {
-            let arg_type = env.context.i8_type().ptr_type(AddressSpace::Generic);
-
-            let function_value = crate::llvm::refcounting::build_header_help(
-                env,
-                &fn_name,
-                env.context.bool_type().into(),
-                &[arg_type.into(), arg_type.into()],
-            );
-
-            let kind_id = Attribute::get_named_enum_kind_id("alwaysinline");
-            debug_assert!(kind_id > 0);
-            let attr = env.context.create_enum_attribute(kind_id, 1);
-            function_value.add_attribute(AttributeLoc::Function, attr);
-
-            let entry = env.context.append_basic_block(function_value, "entry");
-            env.builder.position_at_end(entry);
-
-            debug_info_init!(env, function_value);
-
-            let mut it = function_value.get_param_iter();
-            let value_ptr1 = it.next().unwrap().into_pointer_value();
-            let value_ptr2 = it.next().unwrap().into_pointer_value();
-
-            set_name(value_ptr1.into(), Symbol::ARG_1.ident_string(&env.interns));
-            set_name(value_ptr2.into(), Symbol::ARG_2.ident_string(&env.interns));
-
-            let value_type = basic_type_from_layout(env.arena, env.context, layout, env.ptr_bytes)
-                .ptr_type(AddressSpace::Generic);
-
-            let value_cast1 = env
-                .builder
-                .build_bitcast(value_ptr1, value_type, "load_opaque")
-                .into_pointer_value();
-
-            let value_cast2 = env
-                .builder
-                .build_bitcast(value_ptr2, value_type, "load_opaque")
-                .into_pointer_value();
-
-            let value1 = env.builder.build_load(value_cast1, "load_opaque");
-            let value2 = env.builder.build_load(value_cast2, "load_opaque");
-
-            let result =
-                crate::llvm::compare::generic_eq(env, layout_ids, value1, value2, layout, layout);
-
-            env.builder.build_return(Some(&result));
-
-            function_value
-        }
-    };
-
-    env.builder.position_at_end(block);
-    env.builder
-        .set_current_debug_location(env.context, di_location);
-
-    function_value
-}
-
-fn build_rc_wrapper<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    layout_ids: &mut LayoutIds<'a>,
-    layout: &Layout<'a>,
-    rc_operation: Mode,
-) -> FunctionValue<'ctx> {
-    let block = env.builder.get_insert_block().expect("to be in a function");
-    let di_location = env.builder.get_current_debug_location().unwrap();
-
-    let symbol = Symbol::GENERIC_RC_REF;
-    let fn_name = layout_ids
-        .get(symbol, &layout)
-        .to_symbol_string(symbol, &env.interns);
-
-    let fn_name = match rc_operation {
-        Mode::Inc(n) => format!("{}_inc_{}", fn_name, n),
-        Mode::Dec => format!("{}_dec", fn_name),
-    };
-
-    let function_value = match env.module.get_function(fn_name.as_str()) {
-        Some(function_value) => function_value,
-        None => {
-            let arg_type = env.context.i8_type().ptr_type(AddressSpace::Generic);
-
-            let function_value = crate::llvm::refcounting::build_header_help(
-                env,
-                &fn_name,
-                env.context.void_type().into(),
-                &[arg_type.into()],
-            );
-
-            let kind_id = Attribute::get_named_enum_kind_id("alwaysinline");
-            debug_assert!(kind_id > 0);
-            let attr = env.context.create_enum_attribute(kind_id, 1);
-            function_value.add_attribute(AttributeLoc::Function, attr);
-
-            let entry = env.context.append_basic_block(function_value, "entry");
-            env.builder.position_at_end(entry);
-
-            debug_info_init!(env, function_value);
-
-            let mut it = function_value.get_param_iter();
-            let value_ptr = it.next().unwrap().into_pointer_value();
-
-            set_name(value_ptr.into(), Symbol::ARG_1.ident_string(&env.interns));
-
-            let value_type = basic_type_from_layout(env.arena, env.context, layout, env.ptr_bytes)
-                .ptr_type(AddressSpace::Generic);
-
-            let value_cast = env
-                .builder
-                .build_bitcast(value_ptr, value_type, "load_opaque")
-                .into_pointer_value();
-
-            let value = env.builder.build_load(value_cast, "load_opaque");
-
-            match rc_operation {
-                Mode::Inc(n) => {
-                    increment_refcount_layout(env, function_value, layout_ids, n, value, layout);
-                }
-                Mode::Dec => {
-                    decrement_refcount_layout(env, function_value, layout_ids, value, layout);
-                }
-            }
-
-            env.builder.build_return(None);
-
-            function_value
-        }
-    };
-
-    env.builder.position_at_end(block);
-    env.builder
-        .set_current_debug_location(env.context, di_location);
-
-    function_value
-}
-
 fn dict_symbol_to_zig_dict<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     scope: &Scope<'a, 'ctx>,
diff --git a/compiler/gen/src/llvm/build_hash.rs b/compiler/gen/src/llvm/build_hash.rs
index a5445e1a39..0da80d7fc6 100644
--- a/compiler/gen/src/llvm/build_hash.rs
+++ b/compiler/gen/src/llvm/build_hash.rs
@@ -1,8 +1,7 @@
 use crate::debug_info_init;
+use crate::llvm::bitcode::call_bitcode_fn;
 use crate::llvm::build::Env;
-use crate::llvm::build::{
-    call_bitcode_fn, cast_block_of_memory_to_tag, complex_bitcast, set_name, FAST_CALL_CONV,
-};
+use crate::llvm::build::{cast_block_of_memory_to_tag, complex_bitcast, set_name, FAST_CALL_CONV};
 use crate::llvm::build_str;
 use crate::llvm::convert::basic_type_from_layout;
 use bumpalo::collections::Vec;
diff --git a/compiler/gen/src/llvm/build_list.rs b/compiler/gen/src/llvm/build_list.rs
index 48e7e6797f..c2f3639f0d 100644
--- a/compiler/gen/src/llvm/build_list.rs
+++ b/compiler/gen/src/llvm/build_list.rs
@@ -1,17 +1,21 @@
-use crate::llvm::build::{
-    allocate_with_refcount_help, build_num_binop, cast_basic_basic, Env, InPlace,
+#![allow(clippy::too_many_arguments)]
+use crate::llvm::bitcode::{
+    build_eq_wrapper, build_transform_caller, call_bitcode_fn, call_void_bitcode_fn,
+};
+use crate::llvm::build::{
+    allocate_with_refcount_help, build_num_binop, cast_basic_basic, complex_bitcast, Env, InPlace,
 };
-use crate::llvm::compare::generic_eq;
 use crate::llvm::convert::{basic_type_from_layout, collection, get_ptr_type};
 use crate::llvm::refcounting::{
-    decrement_refcount_layout, increment_refcount_layout, refcount_is_one_comparison,
-    PointerToRefcount,
+    increment_refcount_layout, refcount_is_one_comparison, PointerToRefcount,
 };
 use inkwell::builder::Builder;
 use inkwell::context::Context;
+use inkwell::types::BasicType;
 use inkwell::types::{BasicTypeEnum, PointerType};
 use inkwell::values::{BasicValueEnum, FunctionValue, IntValue, PointerValue, StructValue};
 use inkwell::{AddressSpace, IntPredicate};
+use roc_builtins::bitcode;
 use roc_mono::layout::{Builtin, Layout, LayoutIds, MemoryMode};
 
 /// List.single : a -> List a
@@ -461,8 +465,6 @@ pub fn list_reverse<'a, 'ctx, 'env>(
     list: BasicValueEnum<'ctx>,
     list_layout: &Layout<'a>,
 ) -> BasicValueEnum<'ctx> {
-    use inkwell::types::BasicType;
-
     let builder = env.builder;
     let ctx = env.context;
 
@@ -845,674 +847,423 @@ pub fn list_sum<'a, 'ctx, 'env>(
 }
 
 /// List.walk : List elem, (elem -> accum -> accum), accum -> accum
-#[allow(clippy::too_many_arguments)]
 pub fn list_walk<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
     parent: FunctionValue<'ctx>,
     list: BasicValueEnum<'ctx>,
-    list_layout: &Layout<'a>,
+    element_layout: &Layout<'a>,
     func: BasicValueEnum<'ctx>,
     func_layout: &Layout<'a>,
     default: BasicValueEnum<'ctx>,
     default_layout: &Layout<'a>,
 ) -> BasicValueEnum<'ctx> {
-    let ctx = env.context;
-    let builder = env.builder;
-
-    let list_wrapper = list.into_struct_value();
-    let len = list_len(env.builder, list_wrapper);
-
-    let accum_type = basic_type_from_layout(env.arena, ctx, default_layout, env.ptr_bytes);
-    let accum_alloca = builder.build_alloca(accum_type, "alloca_walk_right_accum");
-    builder.build_store(accum_alloca, default);
-
-    let then_block = ctx.append_basic_block(parent, "then");
-    let cont_block = ctx.append_basic_block(parent, "branchcont");
-
-    let condition = builder.build_int_compare(
-        IntPredicate::UGT,
-        len,
-        ctx.i64_type().const_zero(),
-        "list_non_empty",
-    );
-
-    builder.build_conditional_branch(condition, then_block, cont_block);
-
-    builder.position_at_end(then_block);
-
-    match (func, func_layout) {
-        (BasicValueEnum::PointerValue(func_ptr), Layout::FunctionPointer(_, _)) => {
-            let elem_layout = match list_layout {
-                Layout::Builtin(Builtin::List(_, layout)) => layout,
-                _ => unreachable!("can only fold over a list"),
-            };
-
-            let elem_type = basic_type_from_layout(env.arena, ctx, elem_layout, env.ptr_bytes);
-            let elem_ptr_type = get_ptr_type(&elem_type, AddressSpace::Generic);
-
-            let list_ptr = load_list_ptr(builder, list_wrapper, elem_ptr_type);
-
-            let walk_right_loop = |_, elem: BasicValueEnum<'ctx>| {
-                // load current accumulator
-                let current = builder.build_load(accum_alloca, "retrieve_accum");
-
-                let call_site_value =
-                    builder.build_call(func_ptr, &[elem, current], "#walk_right_func");
-
-                // set the calling convention explicitly for this call
-                call_site_value.set_call_convention(crate::llvm::build::FAST_CALL_CONV);
-
-                let new_current = call_site_value
-                    .try_as_basic_value()
-                    .left()
-                    .unwrap_or_else(|| panic!("LLVM error: Invalid call by pointer."));
-
-                builder.build_store(accum_alloca, new_current);
-            };
-
-            incrementing_elem_loop(
-                builder,
-                ctx,
-                parent,
-                list_ptr,
-                len,
-                "#index",
-                walk_right_loop,
-            );
-        }
-
-        _ => {
-            unreachable!(
-                "Invalid function basic value enum or layout for List.keepIf : {:?}",
-                (func, func_layout)
-            );
-        }
-    }
-
-    builder.build_unconditional_branch(cont_block);
-
-    builder.position_at_end(cont_block);
-
-    builder.build_load(accum_alloca, "load_final_acum")
+    list_walk_generic(
+        env,
+        layout_ids,
+        parent,
+        list,
+        element_layout,
+        func,
+        func_layout,
+        default,
+        default_layout,
+        &bitcode::LIST_WALK,
+    )
 }
 
 /// List.walkBackwards : List elem, (elem -> accum -> accum), accum -> accum
-#[allow(clippy::too_many_arguments)]
 pub fn list_walk_backwards<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
     parent: FunctionValue<'ctx>,
     list: BasicValueEnum<'ctx>,
-    list_layout: &Layout<'a>,
+    element_layout: &Layout<'a>,
     func: BasicValueEnum<'ctx>,
     func_layout: &Layout<'a>,
     default: BasicValueEnum<'ctx>,
     default_layout: &Layout<'a>,
 ) -> BasicValueEnum<'ctx> {
-    let ctx = env.context;
+    list_walk_generic(
+        env,
+        layout_ids,
+        parent,
+        list,
+        element_layout,
+        func,
+        func_layout,
+        default,
+        default_layout,
+        &bitcode::LIST_WALK_BACKWARDS,
+    )
+}
+
+fn list_walk_generic<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    _parent: FunctionValue<'ctx>,
+    list: BasicValueEnum<'ctx>,
+    element_layout: &Layout<'a>,
+    func: BasicValueEnum<'ctx>,
+    func_layout: &Layout<'a>,
+    default: BasicValueEnum<'ctx>,
+    default_layout: &Layout<'a>,
+    zig_function: &str,
+) -> BasicValueEnum<'ctx> {
     let builder = env.builder;
 
-    let list_wrapper = list.into_struct_value();
-    let len = list_len(env.builder, list_wrapper);
+    let u8_ptr = env.context.i8_type().ptr_type(AddressSpace::Generic);
 
-    let accum_type = basic_type_from_layout(env.arena, ctx, default_layout, env.ptr_bytes);
-    let accum_alloca = builder.build_alloca(accum_type, "alloca_walk_right_accum");
-    builder.build_store(accum_alloca, default);
+    let list_i128 = complex_bitcast(env.builder, list, env.context.i128_type().into(), "to_i128");
 
-    let then_block = ctx.append_basic_block(parent, "then");
-    let cont_block = ctx.append_basic_block(parent, "branchcont");
+    let transform_ptr = builder.build_alloca(func.get_type(), "transform_ptr");
+    env.builder.build_store(transform_ptr, func);
 
-    let condition = builder.build_int_compare(
-        IntPredicate::UGT,
-        len,
-        ctx.i64_type().const_zero(),
-        "list_non_empty",
+    let default_ptr = builder.build_alloca(default.get_type(), "default_ptr");
+    env.builder.build_store(default_ptr, default);
+
+    let stepper_caller = build_transform_caller(
+        env,
+        layout_ids,
+        func_layout,
+        &[element_layout.clone(), default_layout.clone()],
+    )
+    .as_global_value()
+    .as_pointer_value();
+
+    let element_width = env
+        .ptr_int()
+        .const_int(element_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let default_width = env
+        .ptr_int()
+        .const_int(default_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let alignment = element_layout.alignment_bytes(env.ptr_bytes);
+    let alignment_iv = env.ptr_int().const_int(alignment as u64, false);
+
+    let result_ptr = env.builder.build_alloca(default.get_type(), "result");
+
+    call_void_bitcode_fn(
+        env,
+        &[
+            list_i128,
+            env.builder
+                .build_bitcast(transform_ptr, u8_ptr, "to_opaque"),
+            stepper_caller.into(),
+            env.builder.build_bitcast(default_ptr, u8_ptr, "to_u8_ptr"),
+            alignment_iv.into(),
+            element_width.into(),
+            default_width.into(),
+            env.builder.build_bitcast(result_ptr, u8_ptr, "to_opaque"),
+        ],
+        zig_function,
     );
 
-    builder.build_conditional_branch(condition, then_block, cont_block);
-
-    builder.position_at_end(then_block);
-
-    match (func, func_layout) {
-        (BasicValueEnum::PointerValue(func_ptr), Layout::FunctionPointer(_, _)) => {
-            let elem_layout = match list_layout {
-                Layout::Builtin(Builtin::List(_, layout)) => layout,
-                _ => unreachable!("can only fold over a list"),
-            };
-
-            let elem_type = basic_type_from_layout(env.arena, ctx, elem_layout, env.ptr_bytes);
-            let elem_ptr_type = get_ptr_type(&elem_type, AddressSpace::Generic);
-
-            let list_ptr = load_list_ptr(builder, list_wrapper, elem_ptr_type);
-
-            let walk_right_loop = |_, elem: BasicValueEnum<'ctx>| {
-                // load current accumulator
-                let current = builder.build_load(accum_alloca, "retrieve_accum");
-
-                let call_site_value =
-                    builder.build_call(func_ptr, &[elem, current], "#walk_right_func");
-
-                // set the calling convention explicitly for this call
-                call_site_value.set_call_convention(crate::llvm::build::FAST_CALL_CONV);
-
-                let new_current = call_site_value
-                    .try_as_basic_value()
-                    .left()
-                    .unwrap_or_else(|| panic!("LLVM error: Invalid call by pointer."));
-
-                builder.build_store(accum_alloca, new_current);
-            };
-
-            decrementing_elem_loop(
-                builder,
-                ctx,
-                parent,
-                list_ptr,
-                len,
-                "#index",
-                walk_right_loop,
-            );
-        }
-
-        _ => {
-            unreachable!(
-                "Invalid function basic value enum or layout for List.keepIf : {:?}",
-                (func, func_layout)
-            );
-        }
-    }
-
-    builder.build_unconditional_branch(cont_block);
-
-    builder.position_at_end(cont_block);
-
-    builder.build_load(accum_alloca, "load_final_acum")
+    env.builder.build_load(result_ptr, "load_result")
 }
 
 /// List.contains : List elem, elem -> Bool
 pub fn list_contains<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
-    parent: FunctionValue<'ctx>,
-    elem: BasicValueEnum<'ctx>,
-    elem_layout: &Layout<'a>,
+    element: BasicValueEnum<'ctx>,
+    element_layout: &Layout<'a>,
     list: BasicValueEnum<'ctx>,
-    list_layout: &Layout<'a>,
 ) -> BasicValueEnum<'ctx> {
-    use inkwell::types::BasicType;
-
     let builder = env.builder;
 
-    let wrapper_struct = list.into_struct_value();
-    let list_elem_layout = match &list_layout {
-        // this pointer will never actually be dereferenced
-        Layout::Builtin(Builtin::EmptyList) => &Layout::Builtin(Builtin::Int64),
-        Layout::Builtin(Builtin::List(_, element_layout)) => element_layout,
-        _ => unreachable!("Invalid layout {:?} in List.contains", list_layout),
-    };
+    let u8_ptr = env.context.i8_type().ptr_type(AddressSpace::Generic);
 
-    let list_elem_type =
-        basic_type_from_layout(env.arena, env.context, list_elem_layout, env.ptr_bytes);
+    let list_i128 = complex_bitcast(env.builder, list, env.context.i128_type().into(), "to_i128");
 
-    let list_ptr = load_list_ptr(
-        builder,
-        wrapper_struct,
-        list_elem_type.ptr_type(AddressSpace::Generic),
-    );
+    let key_ptr = builder.build_alloca(element.get_type(), "key_ptr");
+    env.builder.build_store(key_ptr, element);
 
-    let length = list_len(builder, list.into_struct_value());
+    let element_width = env
+        .ptr_int()
+        .const_int(element_layout.stack_size(env.ptr_bytes) as u64, false);
 
-    list_contains_help(
+    let eq_fn = build_eq_wrapper(env, layout_ids, element_layout);
+
+    call_bitcode_fn(
         env,
-        layout_ids,
-        parent,
-        length,
-        list_ptr,
-        list_elem_layout,
-        elem,
-        elem_layout,
+        &[
+            list_i128,
+            env.builder.build_bitcast(key_ptr, u8_ptr, "to_u8_ptr"),
+            element_width.into(),
+            eq_fn.as_global_value().as_pointer_value().into(),
+        ],
+        bitcode::LIST_CONTAINS,
     )
 }
 
-#[allow(clippy::too_many_arguments)]
-pub fn list_contains_help<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    layout_ids: &mut LayoutIds<'a>,
-    parent: FunctionValue<'ctx>,
-    length: IntValue<'ctx>,
-    source_ptr: PointerValue<'ctx>,
-    list_elem_layout: &Layout<'a>,
-    elem: BasicValueEnum<'ctx>,
-    elem_layout: &Layout<'a>,
-) -> BasicValueEnum<'ctx> {
-    let builder = env.builder;
-    let ctx = env.context;
-
-    let bool_alloca = builder.build_alloca(ctx.bool_type(), "bool_alloca");
-    let index_alloca = builder.build_alloca(ctx.i64_type(), "index_alloca");
-    let next_free_index_alloca = builder.build_alloca(ctx.i64_type(), "next_free_index_alloca");
-
-    builder.build_store(bool_alloca, ctx.bool_type().const_zero());
-    builder.build_store(index_alloca, ctx.i64_type().const_zero());
-    builder.build_store(next_free_index_alloca, ctx.i64_type().const_zero());
-
-    let condition_bb = ctx.append_basic_block(parent, "condition");
-    builder.build_unconditional_branch(condition_bb);
-    builder.position_at_end(condition_bb);
-
-    let index = builder.build_load(index_alloca, "index").into_int_value();
-
-    let condition = builder.build_int_compare(IntPredicate::SGT, length, index, "loopcond");
-
-    let body_bb = ctx.append_basic_block(parent, "body");
-    let cont_bb = ctx.append_basic_block(parent, "cont");
-    builder.build_conditional_branch(condition, body_bb, cont_bb);
-
-    // loop body
-    builder.position_at_end(body_bb);
-
-    let current_elem_ptr = unsafe { builder.build_in_bounds_gep(source_ptr, &[index], "elem_ptr") };
-
-    let current_elem = builder.build_load(current_elem_ptr, "load_elem");
-
-    let has_found = generic_eq(
-        env,
-        layout_ids,
-        current_elem,
-        elem,
-        list_elem_layout,
-        elem_layout,
-    );
-
-    builder.build_store(bool_alloca, has_found.into_int_value());
-
-    let one = ctx.i64_type().const_int(1, false);
-
-    let next_free_index = builder
-        .build_load(next_free_index_alloca, "load_next_free")
-        .into_int_value();
-
-    builder.build_store(
-        next_free_index_alloca,
-        builder.build_int_add(next_free_index, one, "incremented_next_free_index"),
-    );
-
-    builder.build_store(
-        index_alloca,
-        builder.build_int_add(index, one, "incremented_index"),
-    );
-
-    builder.build_conditional_branch(has_found.into_int_value(), cont_bb, condition_bb);
-
-    // continuation
-    builder.position_at_end(cont_bb);
-
-    builder.build_load(bool_alloca, "answer")
-}
-
 /// List.keepIf : List elem, (elem -> Bool) -> List elem
-#[allow(clippy::too_many_arguments)]
 pub fn list_keep_if<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
-    output_inplace: InPlace,
-    parent: FunctionValue<'ctx>,
-    func: BasicValueEnum<'ctx>,
-    func_layout: &Layout<'a>,
+    transform: BasicValueEnum<'ctx>,
+    transform_layout: &Layout<'a>,
     list: BasicValueEnum<'ctx>,
-    list_layout: &Layout<'a>,
+    element_layout: &Layout<'a>,
 ) -> BasicValueEnum<'ctx> {
-    use inkwell::types::BasicType;
-
     let builder = env.builder;
-    let ctx = env.context;
 
-    let wrapper_struct = list.into_struct_value();
-    let (input_inplace, element_layout) = match list_layout.clone() {
-        Layout::Builtin(Builtin::EmptyList) => (
-            InPlace::InPlace,
-            // this pointer will never actually be dereferenced
-            Layout::Builtin(Builtin::Int64),
-        ),
-        Layout::Builtin(Builtin::List(memory_mode, elem_layout)) => (
-            match memory_mode {
-                MemoryMode::Unique => InPlace::InPlace,
-                MemoryMode::Refcounted => InPlace::Clone,
-            },
-            elem_layout.clone(),
-        ),
+    let u8_ptr = env.context.i8_type().ptr_type(AddressSpace::Generic);
 
-        _ => unreachable!("Invalid layout {:?} in List.keepIf", list_layout),
+    let list_i128 = complex_bitcast(env.builder, list, env.context.i128_type().into(), "to_i128");
+
+    let transform_ptr = builder.build_alloca(transform.get_type(), "transform_ptr");
+    env.builder.build_store(transform_ptr, transform);
+
+    let stepper_caller =
+        build_transform_caller(env, layout_ids, transform_layout, &[element_layout.clone()])
+            .as_global_value()
+            .as_pointer_value();
+
+    let element_width = env
+        .ptr_int()
+        .const_int(element_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let alignment = element_layout.alignment_bytes(env.ptr_bytes);
+    let alignment_iv = env.ptr_int().const_int(alignment as u64, false);
+
+    let output = call_bitcode_fn(
+        env,
+        &[
+            list_i128,
+            env.builder
+                .build_bitcast(transform_ptr, u8_ptr, "to_opaque"),
+            stepper_caller.into(),
+            alignment_iv.into(),
+            element_width.into(),
+        ],
+        &bitcode::LIST_KEEP_IF,
+    );
+
+    complex_bitcast(
+        env.builder,
+        output,
+        collection(env.context, env.ptr_bytes).into(),
+        "from_i128",
+    )
+}
+
+/// List.keepOks : List before, (before -> Result after *) -> List after
+pub fn list_keep_oks<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    transform: BasicValueEnum<'ctx>,
+    transform_layout: &Layout<'a>,
+    list: BasicValueEnum<'ctx>,
+    before_layout: &Layout<'a>,
+    after_layout: &Layout<'a>,
+) -> BasicValueEnum<'ctx> {
+    list_keep_result(
+        env,
+        layout_ids,
+        transform,
+        transform_layout,
+        list,
+        before_layout,
+        after_layout,
+        bitcode::LIST_KEEP_OKS,
+    )
+}
+
+/// List.keepErrs : List before, (before -> Result * after) -> List after
+pub fn list_keep_errs<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    transform: BasicValueEnum<'ctx>,
+    transform_layout: &Layout<'a>,
+    list: BasicValueEnum<'ctx>,
+    before_layout: &Layout<'a>,
+    after_layout: &Layout<'a>,
+) -> BasicValueEnum<'ctx> {
+    list_keep_result(
+        env,
+        layout_ids,
+        transform,
+        transform_layout,
+        list,
+        before_layout,
+        after_layout,
+        bitcode::LIST_KEEP_ERRS,
+    )
+}
+
+pub fn list_keep_result<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    transform: BasicValueEnum<'ctx>,
+    transform_layout: &Layout<'a>,
+    list: BasicValueEnum<'ctx>,
+    before_layout: &Layout<'a>,
+    after_layout: &Layout<'a>,
+    op: &str,
+) -> BasicValueEnum<'ctx> {
+    let builder = env.builder;
+
+    let u8_ptr = env.context.i8_type().ptr_type(AddressSpace::Generic);
+
+    let result_layout = match transform_layout {
+        Layout::FunctionPointer(_, ret) => ret,
+        Layout::Closure(_, _, ret) => ret,
+        _ => unreachable!("not a callable layout"),
     };
 
-    let list_type = basic_type_from_layout(env.arena, env.context, &list_layout, env.ptr_bytes);
-    let elem_type = basic_type_from_layout(env.arena, env.context, &element_layout, env.ptr_bytes);
-    let ptr_type = elem_type.ptr_type(AddressSpace::Generic);
+    let list_i128 = complex_bitcast(env.builder, list, env.context.i128_type().into(), "to_i128");
 
-    let list_ptr = load_list_ptr(builder, wrapper_struct, ptr_type);
-    let length = list_len(builder, list.into_struct_value());
+    let transform_ptr = builder.build_alloca(transform.get_type(), "transform_ptr");
+    env.builder.build_store(transform_ptr, transform);
 
-    let zero = ctx.i64_type().const_zero();
+    let stepper_caller =
+        build_transform_caller(env, layout_ids, transform_layout, &[before_layout.clone()])
+            .as_global_value()
+            .as_pointer_value();
 
-    match input_inplace {
-        InPlace::InPlace => {
-            let new_length = list_keep_if_help(
-                env,
-                input_inplace,
-                parent,
-                length,
-                list_ptr,
-                list_ptr,
-                func,
-                func_layout,
-            );
+    let before_width = env
+        .ptr_int()
+        .const_int(before_layout.stack_size(env.ptr_bytes) as u64, false);
 
-            store_list(env, list_ptr, new_length)
-        }
-        InPlace::Clone => {
-            let len_0_block = ctx.append_basic_block(parent, "len_0_block");
-            let len_n_block = ctx.append_basic_block(parent, "len_n_block");
-            let cont_block = ctx.append_basic_block(parent, "cont_block");
+    let after_width = env
+        .ptr_int()
+        .const_int(after_layout.stack_size(env.ptr_bytes) as u64, false);
 
-            let result = builder.build_alloca(list_type, "result");
+    let result_width = env
+        .ptr_int()
+        .const_int(result_layout.stack_size(env.ptr_bytes) as u64, false);
 
-            builder.build_switch(length, len_n_block, &[(zero, len_0_block)]);
+    let alignment = before_layout.alignment_bytes(env.ptr_bytes);
+    let alignment_iv = env.ptr_int().const_int(alignment as u64, false);
 
-            // build block for length 0
-            {
-                builder.position_at_end(len_0_block);
+    let output = call_bitcode_fn(
+        env,
+        &[
+            list_i128,
+            env.builder
+                .build_bitcast(transform_ptr, u8_ptr, "to_opaque"),
+            stepper_caller.into(),
+            alignment_iv.into(),
+            before_width.into(),
+            result_width.into(),
+            after_width.into(),
+        ],
+        op,
+    );
 
-                let new_list = store_list(env, ptr_type.const_zero(), zero);
-
-                builder.build_store(result, new_list);
-                builder.build_unconditional_branch(cont_block);
-            }
-
-            // build block for length > 0
-            {
-                builder.position_at_end(len_n_block);
-
-                let new_list_ptr = allocate_list(env, output_inplace, &element_layout, length);
-
-                let new_length = list_keep_if_help(
-                    env,
-                    InPlace::Clone,
-                    parent,
-                    length,
-                    list_ptr,
-                    new_list_ptr,
-                    func,
-                    func_layout,
-                );
-
-                // store new list pointer there
-                let new_list = store_list(env, new_list_ptr, new_length);
-
-                builder.build_store(result, new_list);
-                builder.build_unconditional_branch(cont_block);
-            }
-
-            builder.position_at_end(cont_block);
-
-            // consume the input list
-            decrement_refcount_layout(env, parent, layout_ids, list, list_layout);
-
-            builder.build_load(result, "load_result")
-        }
-    }
-}
-
-#[allow(clippy::too_many_arguments)]
-pub fn list_keep_if_help<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    _inplace: InPlace,
-    parent: FunctionValue<'ctx>,
-    length: IntValue<'ctx>,
-    source_ptr: PointerValue<'ctx>,
-    dest_ptr: PointerValue<'ctx>,
-    func: BasicValueEnum<'ctx>,
-    func_layout: &Layout<'a>,
-) -> IntValue<'ctx> {
-    match (func, func_layout) {
-        (
-            BasicValueEnum::PointerValue(func_ptr),
-            Layout::FunctionPointer(_, Layout::Builtin(Builtin::Int1)),
-        ) => {
-            let builder = env.builder;
-            let ctx = env.context;
-
-            let index_alloca = builder.build_alloca(ctx.i64_type(), "index_alloca");
-            let next_free_index_alloca =
-                builder.build_alloca(ctx.i64_type(), "next_free_index_alloca");
-
-            builder.build_store(index_alloca, ctx.i64_type().const_zero());
-            builder.build_store(next_free_index_alloca, ctx.i64_type().const_zero());
-
-            // while (length > next_index)
-            let condition_bb = ctx.append_basic_block(parent, "condition");
-            builder.build_unconditional_branch(condition_bb);
-            builder.position_at_end(condition_bb);
-
-            let index = builder.build_load(index_alloca, "index").into_int_value();
-
-            let condition = builder.build_int_compare(IntPredicate::SGT, length, index, "loopcond");
-
-            let body_bb = ctx.append_basic_block(parent, "body");
-            let cont_bb = ctx.append_basic_block(parent, "cont");
-            builder.build_conditional_branch(condition, body_bb, cont_bb);
-
-            // loop body
-            builder.position_at_end(body_bb);
-
-            let elem_ptr = unsafe { builder.build_in_bounds_gep(source_ptr, &[index], "elem_ptr") };
-
-            let elem = builder.build_load(elem_ptr, "load_elem");
-
-            let call_site_value =
-                builder.build_call(func_ptr, env.arena.alloc([elem]), "#keep_if_insert_func");
-
-            // set the calling convention explicitly for this call
-            call_site_value.set_call_convention(crate::llvm::build::FAST_CALL_CONV);
-
-            let should_keep = call_site_value
-                .try_as_basic_value()
-                .left()
-                .unwrap_or_else(|| panic!("LLVM error: Invalid call by pointer."))
-                .into_int_value();
-
-            let filter_pass_bb = ctx.append_basic_block(parent, "loop");
-            let after_filter_pass_bb = ctx.append_basic_block(parent, "after_loop");
-
-            let one = ctx.i64_type().const_int(1, false);
-
-            builder.build_conditional_branch(should_keep, filter_pass_bb, after_filter_pass_bb);
-            builder.position_at_end(filter_pass_bb);
-
-            let next_free_index = builder
-                .build_load(next_free_index_alloca, "load_next_free")
-                .into_int_value();
-
-            // TODO if next_free_index equals index, and we are mutating in place,
-            // then maybe we should not write this value back into memory
-            let dest_elem_ptr = unsafe {
-                builder.build_in_bounds_gep(dest_ptr, &[next_free_index], "dest_elem_ptr")
-            };
-
-            builder.build_store(dest_elem_ptr, elem);
-
-            builder.build_store(
-                next_free_index_alloca,
-                builder.build_int_add(next_free_index, one, "incremented_next_free_index"),
-            );
-
-            builder.build_unconditional_branch(after_filter_pass_bb);
-            builder.position_at_end(after_filter_pass_bb);
-
-            builder.build_store(
-                index_alloca,
-                builder.build_int_add(index, one, "incremented_index"),
-            );
-
-            builder.build_unconditional_branch(condition_bb);
-
-            // continuation
-            builder.position_at_end(cont_bb);
-
-            builder
-                .build_load(next_free_index_alloca, "new_length")
-                .into_int_value()
-        }
-        _ => unreachable!(
-            "Invalid function basic value enum or layout for List.keepIf : {:?}",
-            (func, func_layout)
-        ),
-    }
+    complex_bitcast(
+        env.builder,
+        output,
+        collection(env.context, env.ptr_bytes).into(),
+        "from_i128",
+    )
 }
 
 /// List.map : List before, (before -> after) -> List after
-macro_rules! list_map_help {
-    ($env:expr, $layout_ids:expr, $inplace:expr, $parent:expr, $func:expr, $func_layout:expr, $list:expr, $list_layout:expr, $function_ptr:expr, $function_return_layout: expr, $closure_info:expr) => {{
-        let layout_ids = $layout_ids;
-        let inplace = $inplace;
-        let parent = $parent;
-        let func = $func;
-        let func_layout = $func_layout;
-        let list = $list;
-        let list_layout = $list_layout;
-        let function_ptr = $function_ptr;
-        let function_return_layout = $function_return_layout;
-        let closure_info : Option<(&Layout, BasicValueEnum)> = $closure_info;
-
-
-        let non_empty_fn = |elem_layout: &Layout<'a>,
-                            len: IntValue<'ctx>,
-                            list_wrapper: StructValue<'ctx>| {
-            let ctx = $env.context;
-            let builder = $env.builder;
-
-            let ret_list_ptr = allocate_list($env, inplace, function_return_layout, len);
-
-            let elem_type = basic_type_from_layout($env.arena, ctx, elem_layout, $env.ptr_bytes);
-            let ptr_type = get_ptr_type(&elem_type, AddressSpace::Generic);
-
-            let list_ptr = load_list_ptr(builder, list_wrapper, ptr_type);
-
-            let list_loop = |index, before_elem| {
-                increment_refcount_layout($env, parent, layout_ids, 1, before_elem, elem_layout);
-
-                let arguments = match closure_info {
-                    Some((closure_data_layout, closure_data)) => {
-                        increment_refcount_layout( $env, parent, layout_ids, 1, closure_data, closure_data_layout);
-
-                        bumpalo::vec![in $env.arena; before_elem, closure_data]
-                    }
-                    None => bumpalo::vec![in $env.arena; before_elem],
-                };
-
-
-                let call_site_value = builder.build_call(function_ptr, &arguments, "map_func");
-
-                // set the calling convention explicitly for this call
-                call_site_value.set_call_convention(crate::llvm::build::FAST_CALL_CONV);
-
-                let after_elem = call_site_value
-                    .try_as_basic_value()
-                    .left()
-                    .unwrap_or_else(|| panic!("LLVM error: Invalid call by pointer."));
-
-                // The pointer to the element in the mapped-over list
-                let after_elem_ptr = unsafe {
-                    builder.build_in_bounds_gep(ret_list_ptr, &[index], "load_index_after_list")
-                };
-
-                // Mutate the new array in-place to change the element.
-                builder.build_store(after_elem_ptr, after_elem);
-            };
-
-            incrementing_elem_loop(builder, ctx, parent, list_ptr, len, "#index", list_loop);
-
-            let result = store_list($env, ret_list_ptr, len);
-
-            // decrement the input list and function (if it's a closure)
-            decrement_refcount_layout($env, parent, layout_ids, list, list_layout);
-            decrement_refcount_layout($env, parent, layout_ids, func, func_layout);
-
-            if let Some((closure_data_layout, closure_data))  = closure_info  {
-                decrement_refcount_layout( $env, parent, layout_ids, closure_data, closure_data_layout);
-            }
-
-            result
-        };
-
-        if_list_is_not_empty($env, parent, non_empty_fn, list, list_layout, "List.map")
-    }};
-}
-
-/// List.map : List before, (before -> after) -> List after
-#[allow(clippy::too_many_arguments)]
 pub fn list_map<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
-    inplace: InPlace,
-    parent: FunctionValue<'ctx>,
-    func: BasicValueEnum<'ctx>,
-    func_layout: &Layout<'a>,
+    transform: BasicValueEnum<'ctx>,
+    transform_layout: &Layout<'a>,
     list: BasicValueEnum<'ctx>,
-    list_layout: &Layout<'a>,
+    element_layout: &Layout<'a>,
 ) -> BasicValueEnum<'ctx> {
-    match (func, func_layout) {
-        (BasicValueEnum::PointerValue(func_ptr), Layout::FunctionPointer(_, ret_elem_layout)) => {
-            list_map_help!(
-                env,
-                layout_ids,
-                inplace,
-                parent,
-                func,
-                func_layout,
-                list,
-                list_layout,
-                func_ptr,
-                ret_elem_layout,
-                None
-            )
-        }
-        (
-            BasicValueEnum::StructValue(ptr_and_data),
-            Layout::Closure(_, closure_layout, ret_elem_layout),
-        ) => {
-            let builder = env.builder;
+    list_map_generic(
+        env,
+        layout_ids,
+        transform,
+        transform_layout,
+        list,
+        element_layout,
+        bitcode::LIST_MAP,
+        &[element_layout.clone()],
+    )
+}
 
-            let func_ptr = builder
-                .build_extract_value(ptr_and_data, 0, "function_ptr")
-                .unwrap()
-                .into_pointer_value();
+/// List.mapWithIndex : List before, (Nat, before -> after) -> List after
+pub fn list_map_with_index<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    transform: BasicValueEnum<'ctx>,
+    transform_layout: &Layout<'a>,
+    list: BasicValueEnum<'ctx>,
+    element_layout: &Layout<'a>,
+) -> BasicValueEnum<'ctx> {
+    list_map_generic(
+        env,
+        layout_ids,
+        transform,
+        transform_layout,
+        list,
+        element_layout,
+        bitcode::LIST_MAP_WITH_INDEX,
+        &[Layout::Builtin(Builtin::Usize), element_layout.clone()],
+    )
+}
 
-            let closure_data = builder
-                .build_extract_value(ptr_and_data, 1, "closure_data")
-                .unwrap();
+fn list_map_generic<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    transform: BasicValueEnum<'ctx>,
+    transform_layout: &Layout<'a>,
+    list: BasicValueEnum<'ctx>,
+    element_layout: &Layout<'a>,
+    op: &str,
+    argument_layouts: &[Layout<'a>],
+) -> BasicValueEnum<'ctx> {
+    let builder = env.builder;
 
-            let closure_data_layout = closure_layout.as_block_of_memory_layout();
+    let return_layout = match transform_layout {
+        Layout::FunctionPointer(_, ret) => ret,
+        Layout::Closure(_, _, ret) => ret,
+        _ => unreachable!("not a callable layout"),
+    };
 
-            list_map_help!(
-                env,
-                layout_ids,
-                inplace,
-                parent,
-                func,
-                func_layout,
-                list,
-                list_layout,
-                func_ptr,
-                ret_elem_layout,
-                Some((&closure_data_layout, closure_data))
-            )
-        }
-        _ => {
-            unreachable!(
-                "Invalid function basic value enum or layout for List.map : {:?}",
-                (func, func_layout)
-            );
-        }
-    }
+    let u8_ptr = env.context.i8_type().ptr_type(AddressSpace::Generic);
+
+    let list_i128 = complex_bitcast(env.builder, list, env.context.i128_type().into(), "to_i128");
+
+    let transform_ptr = builder.build_alloca(transform.get_type(), "transform_ptr");
+    env.builder.build_store(transform_ptr, transform);
+
+    let stepper_caller =
+        build_transform_caller(env, layout_ids, transform_layout, argument_layouts)
+            .as_global_value()
+            .as_pointer_value();
+
+    let old_element_width = env
+        .ptr_int()
+        .const_int(element_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let new_element_width = env
+        .ptr_int()
+        .const_int(return_layout.stack_size(env.ptr_bytes) as u64, false);
+
+    let alignment = element_layout.alignment_bytes(env.ptr_bytes);
+    let alignment_iv = env.ptr_int().const_int(alignment as u64, false);
+
+    let output = call_bitcode_fn(
+        env,
+        &[
+            list_i128,
+            env.builder
+                .build_bitcast(transform_ptr, u8_ptr, "to_opaque"),
+            stepper_caller.into(),
+            alignment_iv.into(),
+            old_element_width.into(),
+            new_element_width.into(),
+        ],
+        op,
+    );
+
+    complex_bitcast(
+        env.builder,
+        output,
+        collection(env.context, env.ptr_bytes).into(),
+        "from_i128",
+    )
 }
 
 /// List.concat : List elem, List elem -> List elem
@@ -1856,59 +1607,6 @@ where
     index_alloca
 }
 
-// This function checks if the list is empty, and
-// if it is, it returns an empty list, and if not
-// it runs whatever code is passed in under `build_non_empty`
-// This is the avoid allocating memory if the list is empty.
-fn if_list_is_not_empty<'a, 'ctx, 'env, 'b, NonEmptyFn>(
-    env: &Env<'a, 'ctx, 'env>,
-    parent: FunctionValue<'ctx>,
-    mut build_non_empty: NonEmptyFn,
-    list: BasicValueEnum<'ctx>,
-    list_layout: &Layout<'a>,
-    list_fn_name: &str,
-) -> BasicValueEnum<'ctx>
-where
-    NonEmptyFn: FnMut(&Layout<'a>, IntValue<'ctx>, StructValue<'ctx>) -> BasicValueEnum<'ctx>,
-{
-    match list_layout {
-        Layout::Builtin(Builtin::EmptyList) => empty_list(env),
-
-        Layout::Builtin(Builtin::List(_, elem_layout)) => {
-            let builder = env.builder;
-            let ctx = env.context;
-
-            let wrapper_struct = list.into_struct_value();
-
-            let len = list_len(builder, wrapper_struct);
-
-            // list_len > 0
-            let comparison = builder.build_int_compare(
-                IntPredicate::UGT,
-                len,
-                ctx.i64_type().const_int(0, false),
-                "greaterthanzero",
-            );
-
-            let build_empty = || empty_list(env);
-
-            let struct_type = collection(ctx, env.ptr_bytes);
-
-            build_basic_phi2(
-                env,
-                parent,
-                comparison,
-                || build_non_empty(elem_layout, len, wrapper_struct),
-                build_empty,
-                BasicTypeEnum::StructType(struct_type),
-            )
-        }
-        _ => {
-            unreachable!("Invalid List layout for {} {:?}", list_fn_name, list_layout);
-        }
-    }
-}
-
 pub fn build_basic_phi2<'a, 'ctx, 'env, PassFn, FailFn>(
     env: &Env<'a, 'ctx, 'env>,
     parent: FunctionValue<'ctx>,
diff --git a/compiler/gen/src/llvm/build_str.rs b/compiler/gen/src/llvm/build_str.rs
index 3b1fd0dbac..2ab140db34 100644
--- a/compiler/gen/src/llvm/build_str.rs
+++ b/compiler/gen/src/llvm/build_str.rs
@@ -1,6 +1,5 @@
-use crate::llvm::build::{
-    call_bitcode_fn, call_void_bitcode_fn, complex_bitcast, Env, InPlace, Scope,
-};
+use crate::llvm::bitcode::{call_bitcode_fn, call_void_bitcode_fn};
+use crate::llvm::build::{complex_bitcast, Env, InPlace, Scope};
 use crate::llvm::build_list::{allocate_list, store_list};
 use crate::llvm::convert::collection;
 use inkwell::builder::Builder;
@@ -274,6 +273,19 @@ pub fn str_from_int<'a, 'ctx, 'env>(
     zig_str_to_struct(env, zig_result).into()
 }
 
+/// Str.fromInt : Int -> Str
+pub fn str_from_float<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    scope: &Scope<'a, 'ctx>,
+    int_symbol: Symbol,
+) -> BasicValueEnum<'ctx> {
+    let float = load_symbol(scope, &int_symbol);
+
+    let zig_result = call_bitcode_fn(env, &[float], &bitcode::STR_FROM_FLOAT).into_struct_value();
+
+    zig_str_to_struct(env, zig_result).into()
+}
+
 /// Str.equal : Str, Str -> Bool
 pub fn str_equal<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
diff --git a/compiler/gen/src/llvm/compare.rs b/compiler/gen/src/llvm/compare.rs
index ec1f0ebd05..cd733b3bd1 100644
--- a/compiler/gen/src/llvm/compare.rs
+++ b/compiler/gen/src/llvm/compare.rs
@@ -366,7 +366,6 @@ fn build_list_eq<'a, 'ctx, 'env>(
     list2: StructValue<'ctx>,
     when_recursive: WhenRecursive<'a>,
 ) -> BasicValueEnum<'ctx> {
-    dbg!("list", &when_recursive);
     let block = env.builder.get_insert_block().expect("to be in a function");
     let di_location = env.builder.get_current_debug_location().unwrap();
 
diff --git a/compiler/gen/src/llvm/convert.rs b/compiler/gen/src/llvm/convert.rs
index f5a319aaf8..9cefbc3fbc 100644
--- a/compiler/gen/src/llvm/convert.rs
+++ b/compiler/gen/src/llvm/convert.rs
@@ -283,6 +283,10 @@ pub fn dict(ctx: &Context, ptr_bytes: u32) -> StructType<'_> {
     )
 }
 
+pub fn dict_ptr(ctx: &Context, ptr_bytes: u32) -> PointerType<'_> {
+    dict(ctx, ptr_bytes).ptr_type(AddressSpace::Generic)
+}
+
 pub fn ptr_int(ctx: &Context, ptr_bytes: u32) -> IntType<'_> {
     match ptr_bytes {
         1 => ctx.i8_type(),
diff --git a/compiler/gen/src/llvm/mod.rs b/compiler/gen/src/llvm/mod.rs
index 1d4e0bb4fd..195cbc9c98 100644
--- a/compiler/gen/src/llvm/mod.rs
+++ b/compiler/gen/src/llvm/mod.rs
@@ -1,3 +1,4 @@
+pub mod bitcode;
 pub mod build;
 pub mod build_dict;
 pub mod build_hash;
diff --git a/compiler/gen/tests/gen_compare.rs b/compiler/gen/tests/gen_compare.rs
index 2a119d4869..10bca099ee 100644
--- a/compiler/gen/tests/gen_compare.rs
+++ b/compiler/gen/tests/gen_compare.rs
@@ -416,4 +416,40 @@ mod gen_num {
             bool
         );
     }
+
+    #[test]
+    fn list_eq_empty() {
+        assert_evals_to!("[] == []", true, bool);
+        assert_evals_to!("[] != []", false, bool);
+    }
+
+    #[test]
+    fn list_eq_by_length() {
+        assert_evals_to!("[1] == []", false, bool);
+        assert_evals_to!("[] == [1]", false, bool);
+    }
+
+    #[test]
+    fn list_eq_compare_pointwise() {
+        assert_evals_to!("[1] == [1]", true, bool);
+        assert_evals_to!("[2] == [1]", false, bool);
+    }
+
+    #[test]
+    fn list_eq_nested() {
+        assert_evals_to!("[[1]] == [[1]]", true, bool);
+        assert_evals_to!("[[2]] == [[1]]", false, bool);
+    }
+
+    #[test]
+    fn list_neq_compare_pointwise() {
+        assert_evals_to!("[1] != [1]", false, bool);
+        assert_evals_to!("[2] != [1]", true, bool);
+    }
+
+    #[test]
+    fn list_neq_nested() {
+        assert_evals_to!("[[1]] != [[1]]", false, bool);
+        assert_evals_to!("[[2]] != [[1]]", true, bool);
+    }
 }
diff --git a/compiler/gen/tests/gen_dict.rs b/compiler/gen/tests/gen_dict.rs
index b837a886c0..2e2df034ca 100644
--- a/compiler/gen/tests/gen_dict.rs
+++ b/compiler/gen/tests/gen_dict.rs
@@ -305,4 +305,218 @@ mod gen_dict {
             &[RocStr]
         );
     }
+
+    #[test]
+    fn unit_values() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                myDict : Dict I64 {}
+                myDict =
+                    Dict.empty
+                        |> Dict.insert 0 {}
+                        |> Dict.insert 1 {}
+                        |> Dict.insert 2 {}
+                        |> Dict.insert 3 {}
+
+                Dict.len myDict
+                "#
+            ),
+            4,
+            i64
+        );
+    }
+
+    #[test]
+    fn singleton() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                myDict : Dict I64 {}
+                myDict =
+                    Dict.singleton 0 {}
+
+                Dict.len myDict
+                "#
+            ),
+            1,
+            i64
+        );
+    }
+
+    #[test]
+    fn union() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                myDict : Dict I64 {}
+                myDict =
+                    Dict.union (Dict.singleton 0 {}) (Dict.singleton 1 {})
+
+                Dict.len myDict
+                "#
+            ),
+            2,
+            i64
+        );
+    }
+
+    #[test]
+    fn union_prefer_first() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                myDict : Dict I64 I64
+                myDict =
+                    Dict.union (Dict.singleton 0 100) (Dict.singleton 0 200)
+
+                Dict.values myDict
+                "#
+            ),
+            &[100],
+            &[i64]
+        );
+    }
+
+    #[test]
+    fn intersection() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                dict1 : Dict I64 {}
+                dict1 = 
+                    Dict.empty
+                        |> Dict.insert 1 {}
+                        |> Dict.insert 2 {}
+                        |> Dict.insert 3 {}
+                        |> Dict.insert 4 {}
+                        |> Dict.insert 5 {}
+
+                dict2 : Dict I64 {}
+                dict2 = 
+                    Dict.empty
+                        |> Dict.insert 0 {}
+                        |> Dict.insert 2 {}
+                        |> Dict.insert 4 {}
+
+                Dict.intersection dict1 dict2 
+                    |> Dict.len 
+                "#
+            ),
+            2,
+            i64
+        );
+    }
+
+    #[test]
+    fn intersection_prefer_first() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                dict1 : Dict I64 I64
+                dict1 = 
+                    Dict.empty
+                        |> Dict.insert 1 1
+                        |> Dict.insert 2 2
+                        |> Dict.insert 3 3
+                        |> Dict.insert 4 4
+                        |> Dict.insert 5 5
+
+                dict2 : Dict I64 I64
+                dict2 = 
+                    Dict.empty
+                        |> Dict.insert 0 100
+                        |> Dict.insert 2 200
+                        |> Dict.insert 4 300
+
+                Dict.intersection dict1 dict2 
+                    |> Dict.values 
+                "#
+            ),
+            &[4, 2],
+            &[i64]
+        );
+    }
+
+    #[test]
+    fn difference() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                dict1 : Dict I64 {}
+                dict1 = 
+                    Dict.empty
+                        |> Dict.insert 1 {}
+                        |> Dict.insert 2 {}
+                        |> Dict.insert 3 {}
+                        |> Dict.insert 4 {}
+                        |> Dict.insert 5 {}
+
+                dict2 : Dict I64 {}
+                dict2 = 
+                    Dict.empty
+                        |> Dict.insert 0 {}
+                        |> Dict.insert 2 {}
+                        |> Dict.insert 4 {}
+
+                Dict.difference dict1 dict2 
+                    |> Dict.len 
+                "#
+            ),
+            3,
+            i64
+        );
+    }
+
+    #[test]
+    fn difference_prefer_first() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                dict1 : Dict I64 I64
+                dict1 = 
+                    Dict.empty
+                        |> Dict.insert 1 1
+                        |> Dict.insert 2 2
+                        |> Dict.insert 3 3
+                        |> Dict.insert 4 4
+                        |> Dict.insert 5 5
+
+                dict2 : Dict I64 I64
+                dict2 = 
+                    Dict.empty
+                        |> Dict.insert 0 100
+                        |> Dict.insert 2 200
+                        |> Dict.insert 4 300
+
+                Dict.difference dict1 dict2 
+                    |> Dict.values 
+                "#
+            ),
+            &[5, 3, 1],
+            &[i64]
+        );
+    }
+
+    #[test]
+    fn walk_sum_keys() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                dict1 : Dict I64 I64
+                dict1 = 
+                    Dict.empty
+                        |> Dict.insert 1 1
+                        |> Dict.insert 2 2
+                        |> Dict.insert 3 3
+                        |> Dict.insert 4 4
+                        |> Dict.insert 5 5
+
+                Dict.walk dict1 (\k, _, a -> k + a) 0 
+                "#
+            ),
+            15,
+            i64
+        );
+    }
 }
diff --git a/compiler/gen/tests/gen_list.rs b/compiler/gen/tests/gen_list.rs
index 403cdbee33..60adc5cf02 100644
--- a/compiler/gen/tests/gen_list.rs
+++ b/compiler/gen/tests/gen_list.rs
@@ -1710,39 +1710,32 @@ mod gen_list {
     }
 
     #[test]
-    fn list_eq_empty() {
-        assert_evals_to!("[] == []", true, bool);
-        assert_evals_to!("[] != []", false, bool);
+    fn list_keep_oks() {
+        assert_evals_to!("List.keepOks [] (\\x -> x)", 0, i64);
+        assert_evals_to!("List.keepOks [1,2] (\\x -> Ok x)", &[1, 2], &[i64]);
+        assert_evals_to!("List.keepOks [1,2] (\\x -> x % 2)", &[1, 0], &[i64]);
+        assert_evals_to!("List.keepOks [Ok 1, Err 2] (\\x -> x)", &[1], &[i64]);
     }
 
     #[test]
-    fn list_eq_by_length() {
-        assert_evals_to!("[1] == []", false, bool);
-        assert_evals_to!("[] == [1]", false, bool);
+    fn list_keep_errs() {
+        assert_evals_to!("List.keepErrs [] (\\x -> x)", 0, i64);
+        assert_evals_to!("List.keepErrs [1,2] (\\x -> Err x)", &[1, 2], &[i64]);
+        assert_evals_to!(
+            "List.keepErrs [0,1,2] (\\x -> x % 0 |> Result.mapErr (\\_ -> 32))",
+            &[32, 32, 32],
+            &[i64]
+        );
+        assert_evals_to!("List.keepErrs [Ok 1, Err 2] (\\x -> x)", &[2], &[i64]);
     }
 
     #[test]
-    fn list_eq_compare_pointwise() {
-        assert_evals_to!("[1] == [1]", true, bool);
-        assert_evals_to!("[2] == [1]", false, bool);
-    }
-
-    #[test]
-    fn list_eq_nested() {
-        assert_evals_to!("[[1]] == [[1]]", true, bool);
-        assert_evals_to!("[[2]] == [[1]]", false, bool);
-    }
-
-    #[test]
-    fn list_neq_compare_pointwise() {
-        assert_evals_to!("[1] != [1]", false, bool);
-        assert_evals_to!("[2] != [1]", true, bool);
-    }
-
-    #[test]
-    fn list_neq_nested() {
-        assert_evals_to!("[[1]] != [[1]]", false, bool);
-        assert_evals_to!("[[2]] != [[1]]", true, bool);
+    fn list_map_with_index() {
+        assert_evals_to!(
+            "List.mapWithIndex [0,0,0] (\\index, x -> index + x)",
+            &[0, 1, 2],
+            &[i64]
+        );
     }
 
     #[test]
diff --git a/compiler/gen/tests/gen_result.rs b/compiler/gen/tests/gen_result.rs
new file mode 100644
index 0000000000..be63bf39ca
--- /dev/null
+++ b/compiler/gen/tests/gen_result.rs
@@ -0,0 +1,111 @@
+#[macro_use]
+extern crate pretty_assertions;
+#[macro_use]
+extern crate indoc;
+
+extern crate bumpalo;
+extern crate inkwell;
+extern crate libc;
+extern crate roc_gen;
+
+#[macro_use]
+mod helpers;
+
+#[cfg(test)]
+mod gen_result {
+
+    #[test]
+    fn with_default() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                result : Result I64 {}
+                result = Ok 2
+
+                Result.withDefault result 0
+                "#
+            ),
+            2,
+            i64
+        );
+
+        assert_evals_to!(
+            indoc!(
+                r#"
+                result : Result I64 {}
+                result = Err {} 
+
+                Result.withDefault result 0
+                "#
+            ),
+            0,
+            i64
+        );
+    }
+
+    #[test]
+    fn result_map() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                result : Result I64 {}
+                result = Ok 2
+
+                result
+                    |> Result.map (\x -> x + 1)
+                    |> Result.withDefault 0
+                "#
+            ),
+            3,
+            i64
+        );
+
+        assert_evals_to!(
+            indoc!(
+                r#"
+                result : Result I64 {}
+                result = Err {} 
+
+                result
+                    |> Result.map (\x -> x + 1)
+                    |> Result.withDefault 0
+                "#
+            ),
+            0,
+            i64
+        );
+    }
+
+    #[test]
+    fn result_map_err() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                result : Result {} I64
+                result = Err 2
+
+                when Result.mapErr result (\x -> x + 1) is
+                    Err n -> n
+                    Ok _ -> 0
+                "#
+            ),
+            3,
+            i64
+        );
+
+        assert_evals_to!(
+            indoc!(
+                r#"
+                result : Result {} I64
+                result = Ok {}
+
+                when Result.mapErr result (\x -> x + 1) is
+                    Err n -> n
+                    Ok _ -> 0
+                "#
+            ),
+            0,
+            i64
+        );
+    }
+}
diff --git a/compiler/gen/tests/gen_set.rs b/compiler/gen/tests/gen_set.rs
new file mode 100644
index 0000000000..012eaf64ab
--- /dev/null
+++ b/compiler/gen/tests/gen_set.rs
@@ -0,0 +1,248 @@
+#[macro_use]
+extern crate pretty_assertions;
+#[macro_use]
+extern crate indoc;
+
+extern crate bumpalo;
+extern crate inkwell;
+extern crate libc;
+extern crate roc_gen;
+
+#[macro_use]
+mod helpers;
+
+#[cfg(test)]
+mod gen_set {
+
+    #[test]
+    fn empty_len() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                Set.len Set.empty
+                "#
+            ),
+            0,
+            usize
+        );
+    }
+
+    #[test]
+    fn singleton_len() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                Set.len (Set.singleton 42)
+                "#
+            ),
+            1,
+            usize
+        );
+    }
+
+    #[test]
+    fn singleton_to_list() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                Set.toList (Set.singleton 42)
+                "#
+            ),
+            &[42],
+            &[i64]
+        );
+
+        assert_evals_to!(
+            indoc!(
+                r#"
+                Set.toList (Set.singleton 1)
+                "#
+            ),
+            &[1],
+            &[i64]
+        );
+
+        assert_evals_to!(
+            indoc!(
+                r#"
+                Set.toList (Set.singleton 1.0)
+                "#
+            ),
+            &[1.0],
+            &[f64]
+        );
+    }
+
+    #[test]
+    fn insert() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                Set.empty
+                    |> Set.insert 0
+                    |> Set.insert 1
+                    |> Set.insert 2
+                    |> Set.toList
+                "#
+            ),
+            &[0, 1, 2],
+            &[i64]
+        );
+    }
+
+    #[test]
+    fn remove() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                Set.empty
+                    |> Set.insert 0
+                    |> Set.insert 1
+                    |> Set.remove 1
+                    |> Set.remove 2
+                    |> Set.toList
+                "#
+            ),
+            &[0],
+            &[i64]
+        );
+    }
+
+    #[test]
+    fn union() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                set1 : Set I64
+                set1 = Set.fromList [1,2]
+
+                set2 : Set I64
+                set2 = Set.fromList [1,3,4] 
+
+                Set.union set1 set2
+                    |> Set.toList
+                "#
+            ),
+            &[4, 2, 3, 1],
+            &[i64]
+        );
+    }
+
+    #[test]
+    fn difference() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                set1 : Set I64
+                set1 = Set.fromList [1,2]
+
+                set2 : Set I64
+                set2 = Set.fromList [1,3,4] 
+
+                Set.difference set1 set2
+                    |> Set.toList
+                "#
+            ),
+            &[2],
+            &[i64]
+        );
+    }
+
+    #[test]
+    fn intersection() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                set1 : Set I64
+                set1 = Set.fromList [1,2]
+
+                set2 : Set I64
+                set2 = Set.fromList [1,3,4] 
+
+                Set.intersection set1 set2
+                    |> Set.toList
+                "#
+            ),
+            &[1],
+            &[i64]
+        );
+    }
+
+    #[test]
+    fn walk_sum() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                Set.walk (Set.fromList [1,2,3]) (\x, y -> x + y) 0
+                "#
+            ),
+            6,
+            i64
+        );
+    }
+
+    #[test]
+    fn contains() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                Set.contains (Set.fromList [1,3,4]) 4
+                "#
+            ),
+            true,
+            bool
+        );
+
+        assert_evals_to!(
+            indoc!(
+                r#"
+                Set.contains (Set.fromList [1,3,4]) 2
+                "#
+            ),
+            false,
+            bool
+        );
+    }
+
+    #[test]
+    fn from_list() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                [1,2,2,3,1,4]
+                    |> Set.fromList
+                    |> Set.toList
+                "#
+            ),
+            &[4, 2, 3, 1],
+            &[i64]
+        );
+
+        assert_evals_to!(
+            indoc!(
+                r#"
+                []
+                    |> Set.fromList
+                    |> Set.toList
+                "#
+            ),
+            &[],
+            &[i64]
+        );
+
+        assert_evals_to!(
+            indoc!(
+                r#"
+                empty : List I64
+                empty = []
+
+                empty
+                    |> Set.fromList
+                    |> Set.toList
+                "#
+            ),
+            &[],
+            &[i64]
+        );
+    }
+}
diff --git a/compiler/gen/tests/gen_str.rs b/compiler/gen/tests/gen_str.rs
index c405fbd6bd..62230dc73a 100644
--- a/compiler/gen/tests/gen_str.rs
+++ b/compiler/gen/tests/gen_str.rs
@@ -592,4 +592,9 @@ mod gen_str {
     fn str_join_comma_single() {
         assert_evals_to!(r#"Str.joinWith ["1"] ", " "#, RocStr::from("1"), RocStr);
     }
+
+    #[test]
+    fn str_from_float() {
+        assert_evals_to!(r#"Str.fromFloat 3.14"#, RocStr::from("3.140000"), RocStr);
+    }
 }
diff --git a/compiler/gen/tests/helpers/eval.rs b/compiler/gen/tests/helpers/eval.rs
index e6f3f962df..61c48332a4 100644
--- a/compiler/gen/tests/helpers/eval.rs
+++ b/compiler/gen/tests/helpers/eval.rs
@@ -275,7 +275,7 @@ pub fn helper<'a>(
                 mode,
             );
 
-            fn_val.print_to_stderr();
+            // fn_val.print_to_stderr();
             // module.print_to_stderr();
 
             panic!(
diff --git a/compiler/load/tests/fixtures/build/app_with_deps/AStar.roc b/compiler/load/tests/fixtures/build/app_with_deps/AStar.roc
index b93c6937f3..64d66c1e78 100644
--- a/compiler/load/tests/fixtures/build/app_with_deps/AStar.roc
+++ b/compiler/load/tests/fixtures/build/app_with_deps/AStar.roc
@@ -42,7 +42,7 @@ cheapestOpen = \costFunction, model ->
                             else
                                 Ok smallestSoFar
 
-    Set.foldl model.openSet folder (Err KeyNotFound)
+    Set.walk model.openSet folder (Err KeyNotFound)
         |> Result.map (\x -> x.position)
 
 
@@ -101,11 +101,11 @@ astar = \costFn, moveFn, goal, model ->
 
                neighbours = moveFn current
 
-               newNeighbours = Set.diff neighbours modelPopped.evaluated
+               newNeighbours = Set.difference neighbours modelPopped.evaluated
 
                modelWithNeighbours = { modelPopped & openSet : Set.union modelPopped.openSet newNeighbours }
 
-               modelWithCosts = Set.foldl newNeighbours (\nb, md -> updateCost current nb md) modelWithNeighbours
+               modelWithCosts = Set.walk newNeighbours (\nb, md -> updateCost current nb md) modelWithNeighbours
 
                astar costFn moveFn goal modelWithCosts
 
diff --git a/compiler/load/tests/fixtures/build/interface_with_deps/AStar.roc b/compiler/load/tests/fixtures/build/interface_with_deps/AStar.roc
index 4ddbb5946b..40932f68dc 100644
--- a/compiler/load/tests/fixtures/build/interface_with_deps/AStar.roc
+++ b/compiler/load/tests/fixtures/build/interface_with_deps/AStar.roc
@@ -42,7 +42,7 @@ cheapestOpen = \costFunction, model ->
                             else
                                 Ok smallestSoFar
 
-    Set.foldl model.openSet folder (Err KeyNotFound)
+    Set.walk model.openSet folder (Err KeyNotFound)
         |> Result.map (\x -> x.position)
 
 
@@ -101,11 +101,11 @@ astar = \costFn, moveFn, goal, model ->
 
                neighbours = moveFn current
 
-               newNeighbours = Set.diff neighbours modelPopped.evaluated
+               newNeighbours = Set.difference neighbours modelPopped.evaluated
 
                modelWithNeighbours = { modelPopped & openSet : Set.union modelPopped.openSet newNeighbours }
 
-               modelWithCosts = Set.foldl newNeighbours (\nb, md -> updateCost current nb md) modelWithNeighbours
+               modelWithCosts = Set.walk newNeighbours (\nb, md -> updateCost current nb md) modelWithNeighbours
 
                astar costFn moveFn goal modelWithCosts
 
diff --git a/compiler/module/src/low_level.rs b/compiler/module/src/low_level.rs
index 3416f0a073..e69fa0dd02 100644
--- a/compiler/module/src/low_level.rs
+++ b/compiler/module/src/low_level.rs
@@ -11,6 +11,7 @@ pub enum LowLevel {
     StrSplit,
     StrCountGraphemes,
     StrFromInt,
+    StrFromFloat,
     ListLen,
     ListGetUnsafe,
     ListSet,
@@ -24,10 +25,13 @@ pub enum LowLevel {
     ListPrepend,
     ListJoin,
     ListMap,
+    ListMapWithIndex,
     ListKeepIf,
     ListWalk,
     ListWalkBackwards,
     ListSum,
+    ListKeepOks,
+    ListKeepErrs,
     DictSize,
     DictEmpty,
     DictInsert,
@@ -36,6 +40,11 @@ pub enum LowLevel {
     DictGetUnsafe,
     DictKeys,
     DictValues,
+    DictUnion,
+    DictIntersection,
+    DictDifference,
+    DictWalk,
+    SetFromList,
     NumAdd,
     NumAddWrap,
     NumAddChecked,
diff --git a/compiler/module/src/symbol.rs b/compiler/module/src/symbol.rs
index 0a745dd6e3..25fb2cb0fb 100644
--- a/compiler/module/src/symbol.rs
+++ b/compiler/module/src/symbol.rs
@@ -749,6 +749,13 @@ define_builtins! {
         17 GENERIC_RC_REF: "#generic_rc_by_ref" // refcount of arbitrary layouts, passed as an opaque pointer
 
         18 GENERIC_EQ: "#generic_eq" // internal function that checks generic equality
+
+        // a user-defined function that we need to capture in a closure
+        // see e.g. Set.walk
+        19 USER_FUNCTION: "#user_function"
+
+        // A caller (wrapper) that we pass to zig for it to be able to call Roc functions
+        20 ZIG_FUNCTION_CALLER: "#zig_function_caller"
     }
     1 NUM: "Num" => {
         0 NUM_NUM: "Num" imported // the Num.Num type alias
@@ -865,6 +872,7 @@ define_builtins! {
         8 STR_STARTS_WITH: "startsWith"
         9 STR_ENDS_WITH: "endsWith"
         10 STR_FROM_INT: "fromInt"
+        11 STR_FROM_FLOAT: "fromFloat"
     }
     4 LIST: "List" => {
         0 LIST_LIST: "List" imported // the List.List type alias
@@ -888,10 +896,15 @@ define_builtins! {
         18 LIST_SUM: "sum"
         19 LIST_WALK: "walk"
         20 LIST_LAST: "last"
+        21 LIST_KEEP_OKS: "keepOks"
+        22 LIST_KEEP_ERRS: "keepErrs"
+        23 LIST_MAP_WITH_INDEX: "mapWithIndex"
     }
     5 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" imported // the Result.Result type alias
         1 RESULT_MAP: "map"
+        2 RESULT_MAP_ERR: "mapErr"
+        3 RESULT_WITH_DEFAULT: "withDefault"
     }
     6 DICT: "Dict" => {
         0 DICT_DICT: "Dict" imported // the Dict.Dict type alias
@@ -910,17 +923,30 @@ define_builtins! {
         9 DICT_CONTAINS: "contains"
         10 DICT_KEYS: "keys"
         11 DICT_VALUES: "values"
+
+        12 DICT_UNION: "union"
+        13 DICT_INTERSECTION: "intersection"
+        14 DICT_DIFFERENCE: "difference"
+
+        15 DICT_WALK: "walk"
+
     }
     7 SET: "Set" => {
         0 SET_SET: "Set" imported // the Set.Set type alias
         1 SET_AT_SET: "@Set" // the Set.@Set private tag
         2 SET_EMPTY: "empty"
         3 SET_SINGLETON: "singleton"
-        4 SET_UNION: "union"
-        5 SET_FOLDL: "foldl"
-        6 SET_INSERT: "insert"
-        7 SET_REMOVE: "remove"
-        8 SET_DIFF: "diff"
+        4 SET_LEN: "len"
+        5 SET_INSERT: "insert"
+        6 SET_REMOVE: "remove"
+        7 SET_UNION: "union"
+        8 SET_DIFFERENCE: "difference"
+        9 SET_INTERSECTION: "intersection"
+        10 SET_TO_LIST: "toList"
+        11 SET_FROM_LIST: "fromList"
+        12 SET_WALK: "walk"
+        13 SET_WALK_USER_FUNCTION: "#walk_user_function"
+        14 SET_CONTAINS: "contains"
     }
 
     num_modules: 8 // Keep this count up to date by hand! (TODO: see the mut_map! macro for how we could determine this count correctly in the macro)
diff --git a/compiler/mono/src/borrow.rs b/compiler/mono/src/borrow.rs
index 6304f1cf64..f142d01368 100644
--- a/compiler/mono/src/borrow.rs
+++ b/compiler/mono/src/borrow.rs
@@ -640,11 +640,11 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[bool] {
         ListPrepend => arena.alloc_slice_copy(&[owned, owned]),
         StrJoinWith => arena.alloc_slice_copy(&[irrelevant, irrelevant]),
         ListJoin => arena.alloc_slice_copy(&[irrelevant]),
-        ListMap => arena.alloc_slice_copy(&[owned, irrelevant]),
-        ListKeepIf => arena.alloc_slice_copy(&[owned, irrelevant]),
+        ListMap | ListMapWithIndex => arena.alloc_slice_copy(&[owned, irrelevant]),
+        ListKeepIf | ListKeepOks | ListKeepErrs => arena.alloc_slice_copy(&[owned, irrelevant]),
         ListContains => arena.alloc_slice_copy(&[borrowed, irrelevant]),
-        ListWalk => arena.alloc_slice_copy(&[borrowed, irrelevant, owned]),
-        ListWalkBackwards => arena.alloc_slice_copy(&[borrowed, irrelevant, owned]),
+        ListWalk => arena.alloc_slice_copy(&[owned, irrelevant, owned]),
+        ListWalkBackwards => arena.alloc_slice_copy(&[owned, irrelevant, owned]),
         ListSum => arena.alloc_slice_copy(&[borrowed]),
 
         // TODO when we have lists with capacity (if ever)
@@ -661,7 +661,7 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[bool] {
             arena.alloc_slice_copy(&[irrelevant])
         }
         StrStartsWith | StrEndsWith => arena.alloc_slice_copy(&[owned, borrowed]),
-        StrFromInt => arena.alloc_slice_copy(&[irrelevant]),
+        StrFromInt | StrFromFloat => arena.alloc_slice_copy(&[irrelevant]),
         Hash => arena.alloc_slice_copy(&[borrowed, irrelevant]),
         DictSize => arena.alloc_slice_copy(&[borrowed]),
         DictEmpty => &[],
@@ -670,5 +670,11 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[bool] {
         DictContains => arena.alloc_slice_copy(&[borrowed, borrowed]),
         DictGetUnsafe => arena.alloc_slice_copy(&[borrowed, borrowed]),
         DictKeys | DictValues => arena.alloc_slice_copy(&[borrowed]),
+        DictUnion | DictDifference | DictIntersection => arena.alloc_slice_copy(&[owned, borrowed]),
+
+        // borrow function argument so we don't have to worry about RC of the closure
+        DictWalk => arena.alloc_slice_copy(&[owned, borrowed, owned]),
+
+        SetFromList => arena.alloc_slice_copy(&[owned]),
     }
 }
diff --git a/compiler/mono/src/ir.rs b/compiler/mono/src/ir.rs
index 6167aa55b7..0b94f7a489 100644
--- a/compiler/mono/src/ir.rs
+++ b/compiler/mono/src/ir.rs
@@ -6180,20 +6180,34 @@ fn call_by_name<'a>(
                             None if assigned.module_id() != proc_name.module_id() => {
                                 add_needed_external(procs, env, original_fn_var, proc_name);
 
-                                debug_assert_eq!(
-                                    arg_layouts.len(),
-                                    field_symbols.len(),
-                                    "scroll up a bit for background"
-                                );
-
-                                let call = self::Call {
-                                    call_type: CallType::ByName {
-                                        name: proc_name,
-                                        ret_layout: ret_layout.clone(),
-                                        full_layout: full_layout.clone(),
-                                        arg_layouts,
-                                    },
-                                    arguments: field_symbols,
+                                let call = if proc_name.module_id() == ModuleId::ATTR {
+                                    // the callable is one of the ATTR::ARG_n symbols
+                                    // we must call those by-pointer
+                                    self::Call {
+                                        call_type: CallType::ByPointer {
+                                            name: proc_name,
+                                            ret_layout: ret_layout.clone(),
+                                            full_layout: full_layout.clone(),
+                                            arg_layouts,
+                                        },
+                                        arguments: field_symbols,
+                                    }
+                                } else {
+                                    debug_assert_eq!(
+                                        arg_layouts.len(),
+                                        field_symbols.len(),
+                                        "scroll up a bit for background {:?}",
+                                        proc_name
+                                    );
+                                    self::Call {
+                                        call_type: CallType::ByName {
+                                            name: proc_name,
+                                            ret_layout: ret_layout.clone(),
+                                            full_layout: full_layout.clone(),
+                                            arg_layouts,
+                                        },
+                                        arguments: field_symbols,
+                                    }
                                 };
 
                                 let result =
diff --git a/compiler/mono/src/layout.rs b/compiler/mono/src/layout.rs
index c35985c63b..668ba8f72f 100644
--- a/compiler/mono/src/layout.rs
+++ b/compiler/mono/src/layout.rs
@@ -1112,6 +1112,7 @@ fn layout_from_flat_type<'a>(
                         other => Ok(other),
                     }
                 }
+                Symbol::SET_SET => dict_layout_from_key_value(env, args[0], Variable::EMPTY_RECORD),
                 _ => {
                     panic!("TODO layout_from_flat_type for {:?}", Apply(symbol, args));
                 }
diff --git a/compiler/solve/tests/solve_expr.rs b/compiler/solve/tests/solve_expr.rs
index d9cc0bccff..2013a74286 100644
--- a/compiler/solve/tests/solve_expr.rs
+++ b/compiler/solve/tests/solve_expr.rs
@@ -3257,7 +3257,7 @@ mod solve_expr {
                                             else
                                                 Ok { position, cost: 0.0 }
 
-                    Set.foldl model.openSet folder (Ok { position: boom {}, cost: 0.0 })
+                    Set.walk model.openSet folder (Ok { position: boom {}, cost: 0.0 })
                         |> Result.map (\x -> x.position)
 
                 astar : Model position -> Result position [ KeyNotFound ]*
diff --git a/examples/benchmarks/AStar.roc b/examples/benchmarks/AStar.roc
new file mode 100644
index 0000000000..78f0022bf5
--- /dev/null
+++ b/examples/benchmarks/AStar.roc
@@ -0,0 +1,128 @@
+interface AStar exposes [ findPath, Model, initialModel ] imports [Quicksort]
+
+findPath = \costFn, moveFn, start, end -> 
+    astar costFn moveFn end (initialModel start)
+
+Model position :
+    {
+        evaluated : Set position,
+        openSet : Set position,
+        costs : Dict position F64,
+        cameFrom : Dict position position
+    }
+
+initialModel : position -> Model position
+initialModel = \start ->
+    {
+        evaluated : Set.empty, 
+        openSet : Set.singleton start,
+        costs : Dict.singleton start 0, 
+        cameFrom : Dict.empty
+    }
+
+
+cheapestOpen : (position -> F64), Model position -> Result position {}
+cheapestOpen = \costFn, model ->
+    model.openSet
+        |> Set.toList
+        |> List.keepOks (\position ->
+            when Dict.get model.costs position is
+                Err _ ->
+                    Err {}
+
+                Ok cost ->
+                    Ok { cost: cost + costFn position, position }
+                    )
+        |> Quicksort.sortBy .cost
+        |> List.first
+        |> Result.map .position
+        |> Result.mapErr (\_ -> {})
+
+
+reconstructPath : Dict position position, position -> List position
+reconstructPath = \cameFrom, goal ->
+    when Dict.get cameFrom goal is
+        Err _ ->
+            []
+
+        Ok next ->
+            List.append (reconstructPath cameFrom next) goal
+
+updateCost : position, position, Model position -> Model position
+updateCost = \current, neighbor, model ->
+    when Dict.get model.costs neighbor is
+        Err _ ->
+            newCameFrom =
+                Dict.insert model.cameFrom neighbor current
+
+            newCosts =
+                Dict.insert model.costs neighbor distanceTo
+
+            distanceTo =
+                reconstructPath newCameFrom neighbor
+                    |> List.len
+                    |> Num.toFloat
+
+            { model & 
+                costs: newCosts,
+                cameFrom: newCameFrom
+            }
+
+        Ok previousDistance ->
+
+            newCameFrom =
+                Dict.insert model.cameFrom neighbor current
+
+            newCosts =
+                Dict.insert model.costs neighbor distanceTo
+
+            distanceTo =
+                reconstructPath newCameFrom neighbor
+                    |> List.len
+                    |> Num.toFloat
+
+            newModel =
+                { model & 
+                    costs: newCosts,
+                    cameFrom: newCameFrom
+                }
+
+
+            if distanceTo < previousDistance then
+                newModel
+
+            else
+                model
+
+astar : (position, position -> F64), (position -> Set position), position, Model position -> Result (List position) {}
+astar = \costFn, moveFn, goal, model ->
+    when cheapestOpen (\source -> costFn source goal) model is
+        Err {} ->
+            Err {}
+
+        Ok current ->
+            if current == goal then
+                Ok (reconstructPath model.cameFrom goal)
+
+            else
+                modelPopped =
+                    { model &
+                        openSet: Set.remove model.openSet current,
+                        evaluated: Set.insert model.evaluated current,
+                    }
+
+                neighbors =
+                    moveFn current
+
+                newNeighbors =
+                    Set.difference neighbors modelPopped.evaluated
+
+                modelWithNeighbors =
+                    { modelPopped &
+                        openSet: Set.union modelPopped.openSet newNeighbors
+                    }
+
+                modelWithCosts =
+                    Set.walk newNeighbors (\n, m -> updateCost current n m) modelWithNeighbors
+
+                astar costFn moveFn goal modelWithCosts
diff --git a/examples/benchmarks/AStarTests.roc b/examples/benchmarks/AStarTests.roc
new file mode 100644
index 0000000000..84ee16b823
--- /dev/null
+++ b/examples/benchmarks/AStarTests.roc
@@ -0,0 +1,46 @@
+app "astar-tests"
+    packages { base: "platform" }
+    imports [base.Task, AStar]
+    provides [ main ] to base
+
+fromList : List a -> Set a
+fromList = \list -> List.walk list (\x, a -> Set.insert a x) Set.empty
+
+
+main : Task.Task {} []
+main =
+    Task.after Task.getInt \n ->
+        when n is
+            1 -> 
+                Task.putLine (showBool test1)
+
+            _ -> 
+                ns = Str.fromInt n
+                Task.putLine "No test \(ns)"
+
+showBool : Bool -> Str
+showBool = \b ->
+    when b is
+        True -> "True"
+        False -> "False"
+
+test1 : Bool
+test1 = 
+    example1 == [3, 4]
+
+example1 : List I64
+example1 =
+    step : I64 -> Set I64
+    step = \n ->
+        when n is
+            1 -> fromList [ 2,3 ]
+            2 -> fromList [4]
+            3 -> fromList [4]
+            _ -> fromList []
+
+    cost : I64, I64 -> F64
+    cost = \_, _ -> 1 
+
+    when AStar.findPath cost step 1 4 is 
+        Ok path -> path
+        Err _ -> []