Make Filter() preserve type. (#365)

* Update slice.go

* add test

slice.go

@@ -8,8 +8,8 @@ import (
 
 // Filter iterates over elements of collection, returning an array of all elements predicate returns truthy for.
 // Play: https://go.dev/play/p/Apjg3WeSi7K
-func Filter[V any](collection []V, predicate func(item V, index int) bool) []V {
-	result := make([]V, 0, len(collection))
+func Filter[V any, VC ~[]V](collection VC, predicate func(item V, index int) bool) VC {
+	result := make(VC, 0, len(collection))
 
 	for i := range collection {
 		if predicate(collection[i], i) {

slice_test.go

@@ -26,6 +26,13 @@ func TestFilter(t *testing.T) {
 	})
 
 	is.Equal(r2, []string{"foo", "bar"})
+
+	type myStrings []string
+	allStrings := myStrings{"", "foo", "bar"}
+	nonempty := Filter(allStrings, func(x string, _ int) bool {
+		return len(x) > 0
+	})
+	is.IsType(nonempty, allStrings, "type preserved")
 }
 
 func TestMap(t *testing.T) {