Merge pull request #4346 from snprajwal/list-walkfrom

builtin(list): add List.walkFrom, List.walkFromUntil

crates/compiler/builtins/roc/List.roc

@@ -29,6 +29,8 @@ interface List
         map3,
         product,
         walkUntil,
+        walkFrom,
+        walkFromUntil,
         range,
         sortWith,
         drop,
@@ -443,6 +445,22 @@ walkBackwardsUntil = \list, initial, func ->
         Continue new -> new
         Break new -> new
 
+## Walks to the end of the list from a specified starting index
+walkFrom : List elem, Nat, state, (state, elem -> state) -> state
+walkFrom = \list, index, state, func ->
+    walkHelp : _, _ -> [Continue _, Break []]
+    walkHelp = \currentState, element -> Continue (func currentState element)
+
+    when List.iterHelp list state walkHelp index (List.len list) is
+        Continue new -> new
+
+## A combination of [List.walkFrom] and [List.walkUntil]
+walkFromUntil : List elem, Nat, state, (state, elem -> [Continue state, Break state]) -> state
+walkFromUntil = \list, index, state, func ->
+    when List.iterHelp list state func index (List.len list) is
+        Continue new -> new
+        Break new -> new
+
 sum : List (Num a) -> Num a
 sum = \list ->
     List.walk list 0 Num.add

crates/compiler/module/src/symbol.rs

@@ -1398,6 +1398,8 @@ define_builtins! {
         75 LIST_WALK_TRY: "walkTry"
         76 LIST_WALK_BACKWARDS_UNTIL: "walkBackwardsUntil"
         77 LIST_COUNT_IF: "countIf"
+        78 LIST_WALK_FROM: "walkFrom"
+        79 LIST_WALK_FROM_UNTIL: "walkFromUntil"
     }
     7 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" exposed_type=true // the Result.Result type alias

crates/compiler/test_gen/src/gen_list.rs

@@ -956,6 +956,12 @@ fn list_walk_until_even_prefix_sum() {
     );
 }
 
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn list_walk_from_sum() {
+    assert_evals_to!(r#"List.walkFrom [1, 2, 3] 1 0 Num.add"#, 5, i64);
+}
+
 #[test]
 #[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
 fn list_keep_if_empty_list_of_int() {
@@ -3482,16 +3488,6 @@ fn list_let_generalization() {
     );
 }
 
-#[test]
-#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
-fn list_walk_backwards_until_sum() {
-    assert_evals_to!(
-        r#"List.walkBackwardsUntil [1, 2] 0 \a,b -> Continue (a + b)"#,
-        3,
-        i64
-    );
-}
-
 #[test]
 #[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
 fn list_walk_backwards_implements_position() {
@@ -3520,6 +3516,16 @@ fn list_walk_backwards_implements_position() {
     );
 }
 
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn list_walk_backwards_until_sum() {
+    assert_evals_to!(
+        r#"List.walkBackwardsUntil [1, 2] 0 \a,b -> Continue (a + b)"#,
+        3,
+        i64
+    );
+}
+
 #[test]
 #[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
 fn list_walk_backwards_until_even_prefix_sum() {
@@ -3537,3 +3543,31 @@ fn list_walk_backwards_until_even_prefix_sum() {
         i64
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn list_walk_from_until_sum() {
+    assert_evals_to!(
+        r#"List.walkFromUntil [1, 2, 3, 4] 2 0 \a,b -> Continue (a + b)"#,
+        7,
+        i64
+    );
+}
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn list_walk_from_even_prefix_sum() {
+    assert_evals_to!(
+        r#"
+        helper = \a, b ->
+            if Num.isEven b then
+                Continue (a + b)
+
+            else
+                Break a
+
+        List.walkFromUntil [2, 4, 8, 9] 1 0 helper"#,
+        4 + 8,
+        i64
+    );
+}