Ensure that Filter methods return empty slice when nothing matches

filter.go

@@ -4,8 +4,8 @@ import "context"
 
 // Filter takes an array of T's and applies the callback fn to each element.
 // If fn returns true, the element is included in the returned collection, if false it is excluded.
-// Example: [1, 2, 3].Filter(<number is odd?>) = [1, 3].
 func Filter[T any](arr []T, fn func(T) bool) (out []T) {
+	out = make([]T, 0)
 	for _, elem := range arr {
 		if fn(elem) {
 			out = append(out, elem)
@@ -20,6 +20,7 @@ func FilterContext[T any](
 	arr []T,
 	fn func(ctx context.Context, elem T) bool,
 ) (out []T) {
+	out = make([]T, 0)
 	for _, elem := range arr {
 		if fn(ctx, elem) {
 			out = append(out, elem)
@@ -33,6 +34,7 @@ func FilterError[T any](
 	arr []T,
 	fn func(elem T) (bool, error),
 ) (out []T, err error) {
+	out = make([]T, 0)
 	for _, elem := range arr {
 		include, err := fn(elem)
 		if err != nil {
@@ -52,6 +54,7 @@ func FilterContextError[T any](
 	arr []T,
 	fn func(ctx context.Context, elem T) (bool, error),
 ) (out []T, err error) {
+	out = make([]T, 0)
 	for _, elem := range arr {
 		include, err := fn(ctx, elem)
 		if err != nil {

filter_test.go

@@ -8,6 +8,26 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func ExampleFilter() {
+	fmt.Println(
+		Filter(
+			[]int{1, 2, 3}, func(item int) bool {
+				return item%2 != 0
+			},
+		),
+	)
+	fmt.Println(
+		Filter(
+			[]int{1, 2, 3}, func(item int) bool {
+				return item > 10
+			},
+		),
+	)
+	// Output:
+	// [1 3]
+	// []
+}
+
 func TestFilterInts(t *testing.T) {
 	ints := []int{1, 2, 3, 4, 5}
 	filtered := Filter(
@@ -18,6 +38,17 @@ func TestFilterInts(t *testing.T) {
 	require.ElementsMatch(t, filtered, []int{2, 4})
 }
 
+func TestFilterInts_NoMatches(t *testing.T) {
+	ints := []int{1, 2, 3}
+	filtered := Filter(
+		ints, func(i int) bool {
+			return i > 10
+		},
+	)
+	require.ElementsMatch(t, filtered, []int{})
+	require.Equal(t, []int{}, filtered)
+}
+
 func TestFilterStrings(t *testing.T) {
 	strings := []string{"a", "bb", "ccc", "dddd"}
 	filtered := Filter(