rearrange builtins

builtins/pop.go

@@ -0,0 +1,27 @@
+package builtins
+
+import "monkey/object"
+
+// Pop ...
+func Pop(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 `pop` must be array, got %s",
+			args[0].Type())
+	}
+
+	arr := args[0].(*object.Array)
+	length := len(arr.Elements)
+
+	if length == 0 {
+		return newError("cannot pop from an empty array")
+	}
+
+	element := arr.Elements[length-1]
+	arr.Elements = arr.Elements[:length-1]
+
+	return element
+}