Add List.count_if

src/build/roc/Builtin.roc

@@ -119,6 +119,19 @@ Builtin :: [].{
 					},
 			)
 
+		count_if : List(a), (a -> Bool) -> U64
+		count_if = |list, predicate|
+			List.fold(
+				list,
+				0,
+				|acc, elem|
+					if predicate(elem) {
+						acc + 1
+					} else {
+						acc
+					},
+			)
+
 		fold : List(item), state, (state, item -> state) -> state
 		fold = |list, init, step| {
 			var $state = init

test/snapshots/repl/list_count_if.md

@@ -0,0 +1,13 @@
+# META
+~~~ini
+description=List.count_if counts elements where predicate returns true
+type=repl
+~~~
+# SOURCE
+~~~roc
+» List.count_if([1, 2, 3, 4, 5], |x| x > 2)
+~~~
+# OUTPUT
+3
+# PROBLEMS
+NIL

test/snapshots/repl/list_count_if_all_match.md

@@ -0,0 +1,13 @@
+# META
+~~~ini
+description=List.count_if returns list length when all elements match
+type=repl
+~~~
+# SOURCE
+~~~roc
+» List.count_if([1, 2, 3, 4, 5], |x| x > 0)
+~~~
+# OUTPUT
+5
+# PROBLEMS
+NIL

test/snapshots/repl/list_count_if_empty.md

@@ -0,0 +1,13 @@
+# META
+~~~ini
+description=List.count_if on empty list returns 0
+type=repl
+~~~
+# SOURCE
+~~~roc
+» List.count_if([], |x| x > 2)
+~~~
+# OUTPUT
+0
+# PROBLEMS
+NIL

test/snapshots/repl/list_count_if_none_match.md

@@ -0,0 +1,13 @@
+# META
+~~~ini
+description=List.count_if returns 0 when no elements match
+type=repl
+~~~
+# SOURCE
+~~~roc
+» List.count_if([1, 2, 3], |x| x > 10)
+~~~
+# OUTPUT
+0
+# PROBLEMS
+NIL