rearrange builtins

builtins/rest.go

@@ -0,0 +1,25 @@
+package builtins
+
+import "monkey/object"
+
+// Rest ...
+func Rest(args ...object.Object) object.Object {
+	if len(args) != 1 {
+		return newError("wrong number of arguments. got=%d, want=1",
+			len(args))
+	}
+	if args[0].Type() != object.ARRAY_OBJ {
+		return newError("argument to `rest` must be array, got %s",
+			args[0].Type())
+	}
+
+	arr := args[0].(*object.Array)
+	length := len(arr.Elements)
+	if length > 0 {
+		newElements := make([]object.Object, length-1, length-1)
+		copy(newElements, arr.Elements[1:length])
+		return &object.Array{Elements: newElements}
+	}
+
+	return nil
+}