builtin(list): add List.countIf

Closes #4174

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

crates/compiler/builtins/roc/List.roc

@@ -62,6 +62,7 @@ interface List
         sortDesc,
         reserve,
         walkBackwardsUntil,
+        countIf,
     ]
     imports [
         Bool.{ Bool },
@@ -528,6 +529,18 @@ dropIf : List a, (a -> Bool) -> List a
 dropIf = \list, predicate ->
     List.keepIf list (\e -> Bool.not (predicate e))
 
+## Run the given function on each element of a list, and return the
+## number of elements for which the function returned `Bool.true`.
+countIf : List a, (a -> Bool) -> Nat
+countIf = \list, predicate ->
+    walkState = \state, elem ->
+        if predicate elem then
+            state + 1
+        else
+            state
+
+    List.walk list 0 walkState
+
 ## This works like [List.map], except only the transformed values that are
 ## wrapped in `Ok` are kept. Any that are wrapped in `Err` are dropped.
 ##

crates/compiler/module/src/symbol.rs

@@ -1377,6 +1377,7 @@ define_builtins! {
         74 LIST_MAP_TRY: "mapTry"
         75 LIST_WALK_TRY: "walkTry"
         76 LIST_WALK_BACKWARDS_UNTIL: "walkBackwardsUntil"
+        77 LIST_COUNT_IF: "countIf"
     }
     7 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" exposed_type=true // the Result.Result type alias

crates/compiler/test_gen/src/gen_list.rs

@@ -1065,6 +1065,92 @@ fn list_keep_if_str_is_hello() {
     );
 }
 
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn list_count_if_empty_list() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            List.countIf [] \_ -> Bool.true
+            "#
+        ),
+        0,
+        usize
+    );
+}
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn list_count_if_always_true_for_non_empty_list() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            alwaysTrue : I64 -> Bool
+            alwaysTrue = \_ ->
+                Bool.true
+
+            oneThroughEight : List I64
+            oneThroughEight =
+                [1,2,3,4,5,6,7,8]
+
+            List.countIf oneThroughEight alwaysTrue
+            "#
+        ),
+        8,
+        usize
+    );
+}
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn list_count_if_always_false_for_non_empty_list() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            alwaysFalse : I64 -> Bool
+            alwaysFalse = \_ ->
+                Bool.false
+
+            List.countIf [1,2,3,4,5,6,7,8] alwaysFalse
+            "#
+        ),
+        0,
+        usize
+    );
+}
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn list_count_if_condition() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            intIsLessThanThree : I64 -> Bool
+            intIsLessThanThree = \i ->
+                i < 3
+
+            List.countIf [1,2,3,4,5,6,7,8] intIsLessThanThree
+            "#
+        ),
+        2,
+        usize
+    );
+}
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn list_count_if_str() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+             List.countIf ["x", "y", "x"] (\x -> x == "x")
+             "#
+        ),
+        2,
+        usize
+    );
+}
+
 #[test]
 #[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
 fn list_map_on_empty_list_with_int_layout() {