I just can't get over my discomfort at Go's take on interfaces. In Go you don't explicitly declare that your type implements an interface, you just pass your type to a function expecting the interface and the Go compiler checks that you have method signatures that match the interface.

This lack of an explicit "implements" statement just bugs me so much. Interfaces can have really subtle semantics; without an explicit "implements" declaration it's unclear what (if any) interface and associated semantics you are promising to implement.

For example, I can easily declare two interfaces (I haven't written any real Go, so sorry if I get the details of this wrong):

type Writable1 interface {
  // Returns the number of bytes successfully written,
  // or "0" or on error or EOF.
  Write(data []byte) int
}

type Writable2 interface {
  // Returns the number of bytes successfully written,
  // or "-1" on error or EOF.
  Write(data []byte) int
}

Now there is no type that can implement both interfaces correctly, but any type that attempts to implement one will also (inadvertently) implement the other, and the Go compiler will be happy to let this happen.

Also, if the interface changes the compiler won't warn you about subclasses that need changing unless you compile the code that tries to use that type by its interface.

I really think explicit is better than implicit when it comes to "implements" declarations. This is a big stumbling block for me ever wanting to use Go.