Add builtin List.walkWithIndexUntil

crates/compiler/builtins/roc/List.roc

@@ -32,6 +32,7 @@ interface List
         product,
         walkWithIndex,
         walkUntil,
+        walkWithIndexUntil,
         walkFrom,
         walkFromUntil,
         range,
@@ -520,6 +521,25 @@ walkWithIndexHelp = \list, state, f, index, length ->
     else
         state
 
+## Like [walkUntil], but at each step the function also receives the index of the current element.
+walkWithIndexUntil : List elem, state, (state, elem, Nat -> [Continue state, Break state]) -> state
+walkWithIndexUntil = \list, state, f ->
+    when walkWithIndexUntilHelp list state f 0 (List.len list) is
+        Continue new -> new
+        Break new -> new
+
+## internal helper
+walkWithIndexUntilHelp : List elem, s, (s, elem, Nat -> [Continue s, Break b]), Nat, Nat -> [Continue s, Break b]
+walkWithIndexUntilHelp = \list, state, f, index, length ->
+    if index < length then
+        when f state (List.getUnsafe list index) index is
+            Continue nextState ->
+                walkWithIndexUntilHelp list nextState f (Num.addWrap index 1) length
+
+            Break b -> Break b
+    else
+        Continue state
+
 ## Note that in other languages, `walkBackwards` is sometimes called `reduceRight`,
 ## `fold`, `foldRight`, or `foldr`.
 walkBackwards : List elem, state, (state, elem -> state) -> state

crates/compiler/module/src/symbol.rs

@@ -1439,6 +1439,7 @@ define_builtins! {
         83 LIST_WALK_WITH_INDEX: "walkWithIndex"
         84 LIST_APPEND_IF_OK: "appendIfOk"
         85 LIST_PREPEND_IF_OK: "prependIfOk"
+        86 LIST_WALK_WITH_INDEX_UNTIL: "walkWithIndexUntil"
     }
     7 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" exposed_type=true // the Result.Result type alias

crates/compiler/solve/tests/solve_expr.rs

@@ -3768,6 +3768,14 @@ mod solve_expr {
         );
     }
 
+    #[test]
+    fn list_walk_with_index_until() {
+        infer_eq_without_problem(
+            indoc!(r#"List.walkWithIndexUntil"#),
+            "List elem, state, (state, elem, Nat -> [Break state, Continue state]) -> state",
+        );
+    }
+
     #[test]
     fn list_drop_at() {
         infer_eq_without_problem(

crates/compiler/test_gen/src/gen_list.rs

@@ -990,6 +990,23 @@ fn list_walk_until_sum() {
     );
 }
 
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm", feature = "gen-dev"))]
+fn list_walk_with_index_until_sum() {
+    assert_evals_to!(
+        r#"
+        List.walkWithIndexUntil [5, 7, 2, 3] 0 (\state, elem, index ->
+            if elem % 2 == 0 then
+                Break state
+            else
+                Continue (elem + index + state)
+        )
+        "#,
+        13,
+        i64
+    );
+}
+
 #[test]
 #[cfg(any(feature = "gen-llvm", feature = "gen-wasm", feature = "gen-dev"))]
 fn list_walk_implements_position() {