setup

just some stuff we need in place for the parser to recognice map2 and to give it a (hardcoded) type

compiler/builtins/src/std.rs

@@ -804,6 +804,19 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         )
     });
 
+    // map2 : List a, List b, (a, b -> c) -> List c
+    add_type(Symbol::LIST_MAP2, {
+        let_tvars! {a, b, c, cvar};
+        top_level_function(
+            vec![
+                list_type(flex(a)),
+                list_type(flex(b)),
+                closure(vec![flex(a), flex(b)], cvar, Box::new(flex(c))),
+            ],
+            Box::new(list_type(flex(c))),
+        )
+    });
+
     // append : List elem, elem -> List elem
     add_type(
         Symbol::LIST_APPEND,

compiler/can/src/builtins.rs

@@ -80,6 +80,7 @@ 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_MAP2 => list_map2,
         LIST_MAP_WITH_INDEX => list_map_with_index,
         LIST_KEEP_IF => list_keep_if,
         LIST_KEEP_OKS => list_keep_oks,
@@ -215,6 +216,7 @@ 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_MAP2 => list_map2,
         Symbol::LIST_MAP_WITH_INDEX => list_map_with_index,
         Symbol::LIST_KEEP_IF => list_keep_if,
         Symbol::LIST_KEEP_OKS => list_keep_oks,
@@ -2113,6 +2115,11 @@ fn list_map_with_index(symbol: Symbol, var_store: &mut VarStore) -> Def {
     lowlevel_2(symbol, LowLevel::ListMapWithIndex, var_store)
 }
 
+/// List.map2 : List a, List b, (a, b -> c) -> List c
+fn list_map2(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_3(symbol, LowLevel::ListMap2, var_store)
+}
+
 /// Dict.hashTestOnly : k, v -> Nat
 pub fn dict_hash_test_only(symbol: Symbol, var_store: &mut VarStore) -> Def {
     lowlevel_2(symbol, LowLevel::Hash, var_store)

compiler/module/src/low_level.rs

@@ -27,6 +27,7 @@ pub enum LowLevel {
     ListPrepend,
     ListJoin,
     ListMap,
+    ListMap2,
     ListMapWithIndex,
     ListKeepIf,
     ListWalk,

compiler/module/src/symbol.rs

@@ -909,6 +909,7 @@ define_builtins! {
         21 LIST_KEEP_OKS: "keepOks"
         22 LIST_KEEP_ERRS: "keepErrs"
         23 LIST_MAP_WITH_INDEX: "mapWithIndex"
+        24 LIST_MAP2: "map2"
     }
     5 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" imported // the Result.Result type alias

compiler/mono/src/borrow.rs

@@ -651,6 +651,7 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[bool] {
         StrJoinWith => arena.alloc_slice_copy(&[borrowed, borrowed]),
         ListJoin => arena.alloc_slice_copy(&[irrelevant]),
         ListMap | ListMapWithIndex => arena.alloc_slice_copy(&[owned, irrelevant]),
+        ListMap2 => arena.alloc_slice_copy(&[owned, owned, irrelevant]),
         ListKeepIf | ListKeepOks | ListKeepErrs => arena.alloc_slice_copy(&[owned, borrowed]),
         ListContains => arena.alloc_slice_copy(&[borrowed, irrelevant]),
         ListWalk => arena.alloc_slice_copy(&[owned, irrelevant, owned]),