add first version of List.dropAt

* adds an implementation with no uniqueness/mutability

compiler/builtins/bitcode/src/list.zig

@@ -833,6 +833,54 @@ pub fn listDrop(
     }
 }
 
+// GIESCH add type inference test
+// GIESCH add unit tests
+// GIESCH do a uniqueness check, and reuse the same array if possible
+// GIESCH figure out where to specify uniqueness of output, update builtins readme
+pub fn listDropAt(
+    list: RocList,
+    alignment: u32,
+    element_width: usize,
+    drop_index: usize,
+    dec: Dec,
+) callconv(.C) RocList {
+    if (list.bytes) |source_ptr| {
+        const size = list.len();
+
+        if (drop_index >= size) {
+            return RocList.empty();
+        }
+
+        if (drop_index < size) {
+            const element = source_ptr + drop_index * element_width;
+            dec(element);
+        }
+
+        // GIESCH is this necessary?
+        if (size < 2 and drop_index == 0) {
+            return RocList.empty();
+        }
+
+        const output = RocList.allocate(alignment, (size - 1), element_width);
+        const target_ptr = output.bytes orelse unreachable;
+
+        const head_size = drop_index * element_width;
+        @memcpy(target_ptr, source_ptr, head_size);
+
+        const tail_target = target_ptr + drop_index * element_width;
+        const tail_source = source_ptr + (drop_index + 1) * element_width;
+        const tail_size = (size - drop_index - 1) * element_width;
+        @memcpy(tail_target, tail_source, tail_size);
+
+        // GIESCH what's the difference between this and Dec?
+        utils.decref(list.bytes, size * element_width, alignment);
+
+        return output;
+    } else {
+        return RocList.empty();
+    }
+}
+
 pub fn listRange(width: utils.IntWidth, low: Opaque, high: Opaque) callconv(.C) RocList {
     return switch (width) {
         .U8 => helper1(u8, low, high),

compiler/builtins/bitcode/src/main.zig

@@ -39,6 +39,7 @@ comptime {
     exportListFn(list.listSortWith, "sort_with");
     exportListFn(list.listConcat, "concat");
     exportListFn(list.listDrop, "drop");
+    exportListFn(list.listDropAt, "drop_at");
     exportListFn(list.listSet, "set");
     exportListFn(list.listSetInPlace, "set_in_place");
     exportListFn(list.listSwap, "swap");

compiler/builtins/docs/List.roc

@@ -422,15 +422,18 @@ min : List (Num a) -> Result (Num a) [ ListWasEmpty ]*
 ## If the given index is outside the bounds of the list, returns the original
 ## list unmodified.
 ##
-## To drop the element at a given index, instead of replacing it, see [List.drop].
+## To drop the element at a given index, instead of replacing it, see [List.dropAt].
 set : List elem, Nat, elem -> List elem
 
+# GIESCH figure out if we should add docs for List.drop;
+#   what's the relationship with List.dropFirst, below?
+# GIESCH add docs re: uniqueness and performance
 ## Drops the element at the given index from the list.
 ##
 ## This has no effect if the given index is outside the bounds of the list.
 ##
 ## To replace the element at a given index, instead of dropping it, see [List.set].
-drop : List elem, Nat -> List elem
+dropAt : List elem, Nat -> List elem
 
 ## Adds a new element to the end of the list.
 ##

compiler/builtins/src/bitcode.rs

@@ -63,6 +63,7 @@ pub const LIST_REPEAT: &str = "roc_builtins.list.repeat";
 pub const LIST_APPEND: &str = "roc_builtins.list.append";
 pub const LIST_PREPEND: &str = "roc_builtins.list.prepend";
 pub const LIST_DROP: &str = "roc_builtins.list.drop";
+pub const LIST_DROP_AT: &str = "roc_builtins.list.drop_at";
 pub const LIST_SWAP: &str = "roc_builtins.list.swap";
 pub const LIST_SINGLE: &str = "roc_builtins.list.single";
 pub const LIST_JOIN: &str = "roc_builtins.list.join";

compiler/builtins/src/std.rs

@@ -920,6 +920,13 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         Box::new(list_type(flex(TVAR1))),
     );
 
+    // dropAt : List elem, Nat -> List elem
+    add_top_level_function_type!(
+        Symbol::LIST_DROP_AT,
+        vec![list_type(flex(TVAR1)), nat_type()],
+        Box::new(list_type(flex(TVAR1))),
+    );
+
     // swap : List elem, Nat, Nat -> List elem
     add_top_level_function_type!(
         Symbol::LIST_SWAP,

compiler/can/src/builtins.rs

@@ -87,6 +87,7 @@ pub fn builtin_defs_map(symbol: Symbol, var_store: &mut VarStore) -> Option<Def>
         LIST_MAP2 => list_map2,
         LIST_MAP3 => list_map3,
         LIST_DROP => list_drop,
+        LIST_DROP_AT => list_drop_at,
         LIST_SWAP => list_swap,
         LIST_MAP_WITH_INDEX => list_map_with_index,
         LIST_KEEP_IF => list_keep_if,
@@ -1979,6 +1980,29 @@ fn list_drop(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
+/// List.dropAt : List elem, Nat -> List elem
+fn list_drop_at(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let list_var = var_store.fresh();
+    let index_var = var_store.fresh();
+
+    let body = RunLowLevel {
+        op: LowLevel::ListDropAt,
+        args: vec![
+            (list_var, Var(Symbol::ARG_1)),
+            (index_var, Var(Symbol::ARG_2)),
+        ],
+        ret_var: list_var,
+    };
+
+    defn(
+        symbol,
+        vec![(list_var, Symbol::ARG_1), (index_var, Symbol::ARG_2)],
+        var_store,
+        body,
+        list_var,
+    )
+}
+
 /// List.append : List elem, elem -> List elem
 fn list_append(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let list_var = var_store.fresh();

compiler/gen_llvm/src/llvm/build.rs

@@ -9,9 +9,10 @@ use crate::llvm::build_dict::{
 use crate::llvm::build_hash::generic_hash;
 use crate::llvm::build_list::{
     self, allocate_list, empty_list, empty_polymorphic_list, list_append, list_concat,
-    list_contains, list_drop, list_get_unsafe, list_join, list_keep_errs, list_keep_if,
-    list_keep_oks, list_len, list_map, list_map2, list_map3, list_map_with_index, list_prepend,
-    list_range, list_repeat, list_reverse, list_set, list_single, list_sort_with, list_swap,
+    list_contains, list_drop, list_drop_at, list_get_unsafe, list_join, list_keep_errs,
+    list_keep_if, list_keep_oks, list_len, list_map, list_map2, list_map3, list_map_with_index,
+    list_prepend, list_range, list_repeat, list_reverse, list_set, list_single, list_sort_with,
+    list_swap,
 };
 use crate::llvm::build_str::{
     empty_str, str_concat, str_count_graphemes, str_ends_with, str_from_float, str_from_int,
@@ -5018,6 +5019,27 @@ fn run_low_level<'a, 'ctx, 'env>(
                 _ => unreachable!("Invalid layout {:?} in List.drop", list_layout),
             }
         }
+        ListDropAt => {
+            // List.dropAt : List elem, Nat -> List elem
+            debug_assert_eq!(args.len(), 2);
+
+            let (list, list_layout) = load_symbol_and_layout(scope, &args[0]);
+            let original_wrapper = list.into_struct_value();
+
+            let count = load_symbol(scope, &args[1]);
+
+            match list_layout {
+                Layout::Builtin(Builtin::EmptyList) => empty_list(env),
+                Layout::Builtin(Builtin::List(element_layout)) => list_drop_at(
+                    env,
+                    layout_ids,
+                    original_wrapper,
+                    count.into_int_value(),
+                    element_layout,
+                ),
+                _ => unreachable!("Invalid layout {:?} in List.dropAt", list_layout),
+            }
+        }
         ListPrepend => {
             // List.prepend : List elem, elem -> List elem
             debug_assert_eq!(args.len(), 2);

compiler/gen_llvm/src/llvm/build_list.rs

@@ -297,7 +297,7 @@ pub fn list_swap<'a, 'ctx, 'env>(
     )
 }
 
-/// List.drop : List elem, Nat, Nat -> List elem
+/// List.drop : List elem, Nat -> List elem
 pub fn list_drop<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
@@ -319,6 +319,29 @@ pub fn list_drop<'a, 'ctx, 'env>(
     )
 }
 
+// GIESCH ask about how this calling/linking to compiled zig works
+/// List.dropAt : List elem, Nat -> List elem
+pub fn list_drop_at<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    original_wrapper: StructValue<'ctx>,
+    count: IntValue<'ctx>,
+    element_layout: &Layout<'a>,
+) -> BasicValueEnum<'ctx> {
+    let dec_element_fn = build_dec_wrapper(env, layout_ids, element_layout);
+    call_bitcode_fn_returns_list(
+        env,
+        &[
+            pass_list_cc(env, original_wrapper.into()),
+            env.alignment_intvalue(element_layout),
+            layout_width(env, element_layout),
+            count.into(),
+            dec_element_fn.as_global_value().as_pointer_value().into(),
+        ],
+        bitcode::LIST_DROP_AT,
+    )
+}
+
 /// List.set : List elem, Nat, elem -> List elem
 pub fn list_set<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,

compiler/module/src/low_level.rs

@@ -41,6 +41,7 @@ pub enum LowLevel {
     ListKeepErrs,
     ListSortWith,
     ListDrop,
+    ListDropAt,
     ListSwap,
     DictSize,
     DictEmpty,
@@ -116,13 +117,13 @@ impl LowLevel {
             StrConcat | StrJoinWith | StrIsEmpty | StrStartsWith | StrStartsWithCodePt
             | StrEndsWith | StrSplit | StrCountGraphemes | StrFromInt | StrFromUtf8
             | StrFromUtf8Range | StrToUtf8 | StrRepeat | StrFromFloat | ListLen | ListGetUnsafe
-            | ListSet | ListDrop | ListSingle | ListRepeat | ListReverse | ListConcat
-            | ListContains | ListAppend | ListPrepend | ListJoin | ListRange | ListSwap
-            | DictSize | DictEmpty | DictInsert | DictRemove | DictContains | DictGetUnsafe
-            | DictKeys | DictValues | DictUnion | DictIntersection | DictDifference
-            | SetFromList | NumAdd | NumAddWrap | NumAddChecked | NumSub | NumSubWrap
-            | NumSubChecked | NumMul | NumMulWrap | NumMulChecked | NumGt | NumGte | NumLt
-            | NumLte | NumCompare | NumDivUnchecked | NumRemUnchecked | NumIsMultipleOf
+            | ListSet | ListDrop | ListDropAt | ListSingle | ListRepeat | ListReverse
+            | ListConcat | ListContains | ListAppend | ListPrepend | ListJoin | ListRange
+            | ListSwap | DictSize | DictEmpty | DictInsert | DictRemove | DictContains
+            | DictGetUnsafe | DictKeys | DictValues | DictUnion | DictIntersection
+            | DictDifference | SetFromList | NumAdd | NumAddWrap | NumAddChecked | NumSub
+            | NumSubWrap | NumSubChecked | NumMul | NumMulWrap | NumMulChecked | NumGt | NumGte
+            | NumLt | NumLte | NumCompare | NumDivUnchecked | NumRemUnchecked | NumIsMultipleOf
             | NumAbs | NumNeg | NumSin | NumCos | NumSqrtUnchecked | NumLogUnchecked | NumRound
             | NumToFloat | NumPow | NumCeiling | NumPowInt | NumFloor | NumIsFinite | NumAtan
             | NumAcos | NumAsin | NumBitwiseAnd | NumBitwiseXor | NumBitwiseOr | NumShiftLeftBy

compiler/module/src/symbol.rs

@@ -964,6 +964,7 @@ define_builtins! {
         31 LIST_SORT_WITH: "sortWith"
         32 LIST_DROP: "drop"
         33 LIST_SWAP: "swap"
+        34 LIST_DROP_AT: "dropAt"
     }
     5 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" imported // the Result.Result type alias

compiler/mono/src/borrow.rs

@@ -993,6 +993,7 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[bool] {
         // List.append should own its first argument
         ListAppend => arena.alloc_slice_copy(&[owned, owned]),
         ListDrop => arena.alloc_slice_copy(&[owned, irrelevant]),
+        ListDropAt => arena.alloc_slice_copy(&[owned, irrelevant]),
         ListSwap => arena.alloc_slice_copy(&[owned, irrelevant, irrelevant]),
 
         Eq | NotEq => arena.alloc_slice_copy(&[borrowed, borrowed]),