builtin(list): implement List.walkBackwardsUntil

Signed-off-by: Prajwal S N <prajwalnadig21@gmail.com>

crates/compiler/builtins/roc/List.roc

@@ -61,6 +61,7 @@ interface List
         sortAsc,
         sortDesc,
         reserve,
+        walkBackwardsUntil,
     ]
     imports [
         Bool.{ Bool },
@@ -436,6 +437,13 @@ walkUntil = \list, initial, step ->
         Continue new -> new
         Break new -> new
 
+## Same as [List.walkUntil], but does it from the end of the list instead.
+walkBackwardsUntil : List elem, state, (state, elem -> [Continue state, Break state]) -> state
+walkBackwardsUntil = \list, initial, func ->
+    when List.iterateBackwards list initial func 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

@@ -1374,6 +1374,7 @@ define_builtins! {
         73 LIST_CAPACITY: "capacity"
         74 LIST_MAP_TRY: "mapTry"
         75 LIST_WALK_TRY: "walkTry"
+        76 LIST_WALK_BACKWARDS_UNTIL: "walkBackwardsUntil"
     }
     7 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" exposed_type=true // the Result.Result type alias

crates/compiler/test_gen/src/gen_list.rs

@@ -3395,3 +3395,59 @@ fn list_let_generalization() {
         usize
     );
 }
+
+#[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() {
+    assert_evals_to!(
+        r#"
+        Option a : [Some a, None]
+
+        find : List a, a -> Option Nat
+        find = \list, needle ->
+            findHelp list needle
+                |> .v
+
+        findHelp = \list, needle ->
+            List.walkBackwardsUntil list { n: 0, v: None } \{ n, v }, element ->
+                if element == needle then
+                    Break { n, v: Some n }
+                else
+                    Continue { n: n + 1, v }
+
+        when find [1, 2, 3] 3 is
+            None -> 0
+            Some v -> v
+        "#,
+        0,
+        usize
+    );
+}
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn list_walk_backwards_until_even_prefix_sum() {
+    assert_evals_to!(
+        r#"
+        helper = \a, b ->
+            if Num.isEven b then
+                Continue (a + b)
+
+            else
+                Break a
+
+        List.walkBackwardsUntil [9, 8, 4, 2] 0 helper"#,
+        2 + 4 + 8,
+        i64
+    );
+}