fn: add deep copy interface.

fn/func.go

@@ -0,0 +1,42 @@
+package fn
+
+// Copyable is a generic interface for a type that's able to return a deep copy
+// of itself.
+type Copyable[T any] interface {
+	Copy() T
+}
+
+// CopyAll creates a new slice where each item of the slice is a deep copy of
+// the elements of the input slice.
+func CopyAll[T Copyable[T]](xs []T) []T {
+	newItems := make([]T, len(xs))
+	for i := range xs {
+		newItems[i] = xs[i].Copy()
+	}
+
+	return newItems
+}
+
+// CopyableErr is a generic interface for a type that's able to return a deep copy
+// of itself. This is identical to Copyable, but should be used in cases where
+// the copy method can return an error.
+type CopyableErr[T any] interface {
+	Copy() (T, error)
+}
+
+// CopyAllErr creates a new slice where each item of the slice is a deep copy of
+// the elements of the input slice. This is identical to CopyAll, but should be
+// used in cases where the copy method can return an error.
+func CopyAllErr[T CopyableErr[T]](xs []T) ([]T, error) {
+	var err error
+
+	newItems := make([]T, len(xs))
+	for i := range xs {
+		newItems[i], err = xs[i].Copy()
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return newItems, nil
+}