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15 Jul 2021 - written by @thinkofher

How not to implement a decorator pattern in Go?

Lets assume you have the following interface in your go codebase and you want to implement very popular design pattern called decorator. It’s a great pattern, that allows you to extend existing objects without breaking down current API.

It’s very often my design pattern of choice, when I’m working with some legacy code.

type Doer interface {
    Do(context.Context) (*DoerResult, error)

Probably, you also heard about interface embedding, so why not try it? What can go wrong?

type DoerDecorator struct {

After embedding our Doer interface inside the decorator, wee need to override its interface method. We can do it like below.

func (d *DoerDecorator) Do(ctx context.Context) (*DoerResult, error) {

    return d.Do()

And it’s not a great idea at all. Also: compiler won’t complain about this. But we should understand further what’s happening here.

Our DoerDecorator has got Doer interface embedded in itself, so it doesn’t need refer to specific field when calling Do method. It can be handy very, but right here it’s unforgivable mistake, because it leads to infinite recursive function call.

Do method will call doSomeThingElse function and then call Do method which calls doSomeThingElse and Do method and again and again, forever.

To fix this up we have to replace return d.Do() with return d.Doer.Do() at Do method of DoerDecorator. By doing so we’re telling compiler to call Do method of embedded interface instead of calling it’s own Do method, which leads us to previously mentioned recursive function call.

If you want to get rid of this and similar mistakes from your codebase, think about setting up staticcheck in your project’s CI. You will never catch all mistakes at code review (and it’s fine we’re only humans), so having additional tools that will seek for common coding mistakes is a great addition.

You can read more about infinite recursive call here.