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Comment by zero_shift | Hacker News Reader

Rather than defining all these one-method interfaces, why not specify a function type?

Instead of

    type Saver interface {
        Save(data []byte) error
    }

You could have

   type saves func([]byte) error

Seems less bulky than an interface, more concise to mock too.

It's more effort when you need to "promote" the port / input type to a full interface, but I think that's a reasonable tradeoff to avoid callers of your function constantly creating structs just to hang methods off

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With Go had something similar to Java's `@FunctionalInterface` annotation, where a functions signature (parameters and return type) is implicitly matched against the single abstract method of a functional interface and where existing matching methods of any object can also be used as implementations of functional interfaces.

You can do this in Go by making a type declaration defining a function and then adding a method with the same signature on that type, which calls the function. The Go standard library does exactly this with the `HandlerFunc` type [1].

  // The HandlerFunc type is an adapter to allow the use of
  // ordinary functions as HTTP handlers. If f is a function
  // with the appropriate signature, HandlerFunc(f) is a
  // [Handler] that calls f.
  type HandlerFunc func(ResponseWriter, *Request)
  
  // ServeHTTP calls f(w, r).
  func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
      f(w, r)
  }

[1]: https://cs.opensource.google/go/go/+/refs/tags/go1.25.4:src/...

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