From 004219d4eef5123721c90a0ff4a8d9029f238eed Mon Sep 17 00:00:00 2001
From: Richard Feldman <oss@rtfeldman.com>
Date: Fri, 7 Nov 2025 08:23:06 -0500
Subject: [PATCH] wip

---
 src/build/builtin_compiler/main.zig |  6 ++++++
 src/build/roc/Builtin.roc           |  2 --
 src/builtins/list.zig               | 10 +++++++++
 src/canonicalize/Expression.zig     |  4 ++++
 src/eval/interpreter.zig            | 32 +++++++++++++++++++++++++++++
 5 files changed, 52 insertions(+), 2 deletions(-)

diff --git a/src/build/builtin_compiler/main.zig b/src/build/builtin_compiler/main.zig
index 03bb18df6b..0ae34c46fe 100644
--- a/src/build/builtin_compiler/main.zig
+++ b/src/build/builtin_compiler/main.zig
@@ -149,6 +149,12 @@ fn replaceStrIsEmptyWithLowLevel(env: *ModuleEnv) !std.ArrayList(CIR.Def.Idx) {
     if (env.common.findIdent("Builtin.Str.is_empty")) |str_is_empty_ident| {
         try low_level_map.put(str_is_empty_ident, .str_is_empty);
     }
+    if (env.common.findIdent("Builtin.List.len")) |list_len_ident| {
+        try low_level_map.put(list_len_ident, .list_len);
+    }
+    if (env.common.findIdent("Builtin.List.is_empty")) |list_is_empty_ident| {
+        try low_level_map.put(list_is_empty_ident, .list_is_empty);
+    }
     if (env.common.findIdent("Builtin.Set.is_empty")) |set_is_empty_ident| {
         try low_level_map.put(set_is_empty_ident, .set_is_empty);
     }
diff --git a/src/build/roc/Builtin.roc b/src/build/roc/Builtin.roc
index feb433ccf7..dae6f06fd1 100644
--- a/src/build/roc/Builtin.roc
+++ b/src/build/roc/Builtin.roc
@@ -8,10 +8,8 @@ Builtin := [].{
 
 	List := [ProvidedByCompiler].{
 		len : List(a) -> U64
-		len = |_| 0
 
 		is_empty : List(a) -> Bool
-		is_empty = |_| True
 
 		first : List(a) -> Try(a, [ListWasEmpty])
 		first = |_| Err(ListWasEmpty)
diff --git a/src/builtins/list.zig b/src/builtins/list.zig
index ce32f60886..e9147642d7 100644
--- a/src/builtins/list.zig
+++ b/src/builtins/list.zig
@@ -375,6 +375,16 @@ pub fn listIncref(list: RocList, amount: isize, elements_refcounted: bool) callc
     list.incref(amount, elements_refcounted);
 }
 
+/// Get the number of elements in the list.
+pub fn listLen(list: RocList) callconv(.c) usize {
+    return list.len();
+}
+
+/// Check if the list is empty.
+pub fn listIsEmpty(list: RocList) callconv(.c) bool {
+    return list.isEmpty();
+}
+
 /// Decrement reference count and deallocate when no longer shared.
 pub fn listDecref(
     list: RocList,
diff --git a/src/canonicalize/Expression.zig b/src/canonicalize/Expression.zig
index 3229d1ad7f..a34e08c501 100644
--- a/src/canonicalize/Expression.zig
+++ b/src/canonicalize/Expression.zig
@@ -387,6 +387,10 @@ pub const Expr = union(enum) {
         // String operations
         str_is_empty,
 
+        // List operations
+        list_len,
+        list_is_empty,
+
         // Set operations
         set_is_empty,
 
diff --git a/src/eval/interpreter.zig b/src/eval/interpreter.zig
index 56fa5a9d9b..3499748205 100644
--- a/src/eval/interpreter.zig
+++ b/src/eval/interpreter.zig
@@ -2119,6 +2119,38 @@ pub const Interpreter = struct {
 
                 return try self.makeSimpleBoolValue(result);
             },
+            .list_len => {
+                // List.len : List(a) -> U64
+                // Note: listLen returns usize, but List.len always returns U64.
+                // We need to cast usize -> u64 for 32-bit targets (e.g. wasm32).
+                if (args.len != 1) return error.TypeMismatch;
+
+                const list_arg = args[0];
+                if (list_arg.ptr == null) return error.TypeMismatch;
+
+                const roc_list: *const builtins.list.RocList = @ptrCast(@alignCast(list_arg.ptr.?));
+                const len_usize = builtins.list.listLen(roc_list.*);
+                const len_u64: u64 = @intCast(len_usize);
+
+                const result_layout = layout.Layout.int(.u64);
+                var out = try self.pushRaw(result_layout, 0);
+                out.is_initialized = false;
+                out.setInt(@intCast(len_u64));
+                out.is_initialized = true;
+                return out;
+            },
+            .list_is_empty => {
+                // List.is_empty : List(a) -> Bool
+                if (args.len != 1) return error.TypeMismatch;
+
+                const list_arg = args[0];
+                if (list_arg.ptr == null) return error.TypeMismatch;
+
+                const roc_list: *const builtins.list.RocList = @ptrCast(@alignCast(list_arg.ptr.?));
+                const result = builtins.list.listIsEmpty(roc_list.*);
+
+                return try self.makeSimpleBoolValue(result);
+            },
             .set_is_empty => {
                 // TODO: implement Set.is_empty
                 self.triggerCrash("Set.is_empty not yet implemented", false, roc_ops);