Add plumbing for List.swap

compiler/builtins/bitcode/src/main.zig

@@ -30,6 +30,7 @@ comptime {
     exportListFn(list.listConcat, "concat");
     exportListFn(list.listDrop, "drop");
     exportListFn(list.listSet, "set");
+    exportListFn(list.listSwap, "swap");
 }
 
 // Dict Module

compiler/builtins/src/bitcode.rs

@@ -63,6 +63,7 @@ pub const LIST_CONTAINS: &str = "roc_builtins.list.contains";
 pub const LIST_REPEAT: &str = "roc_builtins.list.repeat";
 pub const LIST_APPEND: &str = "roc_builtins.list.append";
 pub const LIST_DROP: &str = "roc_builtins.list.drop";
+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";
 pub const LIST_RANGE: &str = "roc_builtins.list.range";

compiler/builtins/src/std.rs

@@ -856,6 +856,13 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         Box::new(list_type(flex(TVAR1))),
     );
 
+    // swap : List elem, Nat, Nat -> List elem
+    add_top_level_function_type!(
+        Symbol::LIST_SWAP,
+        vec![list_type(flex(TVAR1)), nat_type(), nat_type()],
+        Box::new(list_type(flex(TVAR1))),
+    );
+
     // prepend : List elem, elem -> List elem
     add_top_level_function_type!(
         Symbol::LIST_PREPEND,

compiler/can/src/builtins.rs

@@ -85,6 +85,7 @@ 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_SWAP => list_swap,
         LIST_MAP_WITH_INDEX => list_map_with_index,
         LIST_KEEP_IF => list_keep_if,
         LIST_KEEP_OKS => list_keep_oks,
@@ -1882,6 +1883,36 @@ fn list_set(symbol: Symbol, var_store: &mut VarStore) -> Def {
         list_ret_var,
     )
 }
+
+/// List.swap : List elem, Nat, Nat -> List elem
+fn list_drop(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let list_var = var_store.fresh();
+    let index1_var = var_store.fresh();
+    let index2_var = var_store.fresh();
+
+    let body = RunLowLevel {
+        op: LowLevel::ListDrop,
+        args: vec![
+            (list_var, Var(Symbol::ARG_1)),
+            (index1_var, Var(Symbol::ARG_2)),
+            (index2_var, Var(Symbol::ARG_3)),
+        ],
+        ret_var: list_var,
+    };
+
+    defn(
+        symbol,
+        vec![
+            (list_var, Symbol::ARG_1),
+            (index1_var, Symbol::ARG_2),
+            (index2_var, Symbol::ARG_3),
+        ],
+        var_store,
+        body,
+        list_var,
+    )
+}
+
 /// List.drop : List elem, Nat -> List elem
 fn list_drop(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let list_var = var_store.fresh();

compiler/gen/src/llvm/build.rs

@@ -8,7 +8,7 @@ use crate::llvm::build_list::{
     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_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,
@@ -4164,6 +4164,27 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             list_append(env, layout.in_place(), original_wrapper, elem, elem_layout)
         }
+        ListSwap => {
+            // List.swap : List elem, Nat, Nat -> List elem
+            debug_assert_eq!(args.len(), 3);
+
+            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_swap(
+                    env,
+                    layout_ids,
+                    original_wrapper,
+                    count.into_int_value(),
+                    element_layout,
+                ),
+                _ => unreachable!("Invalid layout {:?} in List.swap", list_layout),
+            }
+        }
         ListDrop => {
             // List.drop : List elem, Nat -> List elem
             debug_assert_eq!(args.len(), 2);

compiler/gen/src/llvm/build_list.rs

@@ -304,7 +304,28 @@ pub fn list_append<'a, 'ctx, 'env>(
     )
 }
 
-/// List.drop : List elem, Nat -> List elem
+/// List.swap : List elem, Nat, Nat -> List elem
+pub fn list_swap<'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_as_i128(env, original_wrapper.into()),
+            env.alignment_intvalue(&element_layout),
+            layout_width(env, &element_layout),
+            count.into(),
+        ],
+        &bitcode::LIST_SWAP,
+    )
+}
+
+/// List.drop : List elem, Nat, Nat -> List elem
 pub fn list_drop<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,

compiler/module/src/low_level.rs

@@ -135,6 +135,7 @@ impl LowLevel {
             | ListPrepend
             | ListJoin
             | ListRange
+            | ListSwap
             | DictSize
             | DictEmpty
             | DictInsert

compiler/module/src/symbol.rs

@@ -939,6 +939,7 @@ define_builtins! {
         30 LIST_RANGE: "range"
         31 LIST_SORT_WITH: "sortWith"
         32 LIST_DROP: "drop"
+        33 LIST_SWAP: "swap"
     }
     5 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" imported // the Result.Result type alias

compiler/mono/src/borrow.rs

@@ -748,6 +748,7 @@ 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]),
+        ListSwap => arena.alloc_slice_copy(&[owned, irrelevant, irrelevant]),
 
         Eq | NotEq => arena.alloc_slice_copy(&[borrowed, borrowed]),