Add missing test and docstring to Passthrough

Author: BooleanCat

iter/ops/unary.go

@@ -17,6 +17,8 @@ func UnwrapResult[T any](r result.Result[T]) T {
 	return r.Unwrap()
 }
 
+// Passthrough may be used an an operation for iter.Map. It returns the
+// provided value without modification.
 func Passthrough[T any](t T) T {
 	return t
 }

iter/ops/unary_test.go

@@ -38,6 +38,13 @@ func ExampleUnwrapResult() {
 	// Output: [1 3 -6]
 }
 
+func ExamplePassthrough() {
+	numbers := iter.Map[int](iter.Lift([]int{1, 2, 3}), ops.Passthrough[int])
+
+	fmt.Println(iter.Collect[int](numbers))
+	// Output: [1 2 3]
+}
+
 func TestUnwrapOption(t *testing.T) {
 	options := iter.Lift([]option.Option[int]{
 		option.Some(4),