[Groxx]

Both seem like fair options to me. With function scope, you can

    func f() {
      x := ...
      if x.something() {
        x.doSomethingEarlier()
        defer x.cleanup()
      }
      // use x however you like
    }

where scope-based forces you to do stuff like

    func f() {
      x := ...
      if x.something() {
        x.doSomethingEarlier()
        defer x.cleanup()
        // use x however you like
      } else {
        // use x however you like
      }
    }

In a scope-based defer, you'd have to keep all related code in the scope, nesting it another layer deeper / possibly duplicating it.

On the flip-side is of course that this doesn't work like most would probably want in function-scoped:

    for i := 0; i < 4; i++ {
      x := get(i)
      defer x.cleanup()
      x.whatever()
    }

and you're forced to

    for i := 0; i < 4; i++ {
      x := get(i)
      func() {
        defer x.cleanup()
        x.whatever()
      }()
    }

I've seen both of these patterns pretty frequently, in Go and in other languages. Go could, of course, have both a func_defer and a scope_defer, but that doesn't seem like it'd fit with the fairly strong focus on keeping the language feeling small and simple. So they had to pick one, and it can't handle both cases.

[pcwalton]

I've never seen the first pattern in any other language. Aside from downcasts as in your other example, why would you only want to clean up an object if some condition is true? The 99% use case of defer is for resource destruction, which you nearly always want to do in the same scope the object was initialized in (and that observation in fact is what underlies RAII).

[infogulch]

I think function scope is a good default, and wrapping in an anonymous function and calling it (like your last example) is a simple workaround to get the scope_defer behavior. If it was scope based there's nothing you could do to get func_defer behavior.

[Groxx]

Yeah, I generally feel the same way. For fairly simple use, scope is more consistent (all scopes / closures are identical), but func is a bit more flexible if you're willing to pay with simple boilerplate.

I mean, you can convert them into each other. Scoped can do something like this (go+python blended code 'cuz lazy):

    func f(){
      deferred := []
      defer func() { for d in deferred.reverse(): d() }() // plus error handling
      if x.something() {
        deferred.push(func(){ cleanup() });
      }
      // same as func scope
    }

but that's a bit more ridiculous / error-prone (though a helper func is obviously possible) than the equivalent IIFE for func -> scope. More explicit, I suppose, but bleh.

[pcwalton]

It's more explicit, which is a good thing, as it makes the intent clear. This matters if, for example, the function is later refactored to inline into a caller.

[Groxx]

It also allows more flexibility (do you execute them in the order they were enqueued, or in reverse?), more room for errors, confusion between different patterns / lack of consistency across different codebases, etc.

Explicit-all-the-things isn't an unambiguous Good Thing™. If it were, we wouldn't even be discussing this - it's an abstraction, which is less explicit than e.g. building defer out of a list and using GOTO.

[pcwalton]

> If it was scope based there's nothing you could do to get func_defer behavior.

That's clearly false. You could set up a list to hold objects to be disposed (or, more generally, closures to execute) and defer a simple procedure that disposes of all objects in the list. This is in fact what the implementation of defer must do internally.

[ovao]

I think by that he means it wouldn't be possible to get func_defer behavior in the example with a single keyword, or without some form of qualifier. The compiler wouldn't be able to differentiate the behavior.