Type assertion from error to custom error type


I have been declaring sentinel error types, as in https://play.golang.org/p/mhaBt3ehby0. However, it seems that the type assertion from any error (e.g. errA) to ErrB always succeeds, even though errA is an instance of ErrA. I could understand this if I were using type aliases, but surely type ErrA is distinct from type ErrB?

I can fix the problem by doing https://play.golang.org/p/H14SERIJdem, but what is the explanation for the first scenario and is there a better way to do this than using struct




A non-constant value x can be converted to type T in any of these cases:

Explains the first example. But I can’t explain the second one which is based on composition.

Hi Andrew,

In Go, error is an interface type


When you do a type assertion on an interface type, it just checks to see if it satisfies the given interface. That is, (in the example you gave) whether errA satisfies the errB interface. The value of errA is not changed, but the resulting expression is of type errB, and has methods from errB. In this case both errA and errB are aliases for error, so the type assertion really changes nothing. You’ve just succeeded in converting something of type errA to type errB.

Your Println() statements are printing two things that have the same value, so they print the same thing.

Right, thanks, its obvious when I think about it now. ErrB genuinely is a new interface type, but it is an interface type with the same layout as error. I could equally well have defined ErrB like https://play.golang.org/p/PavzI5z4PM5, and the type assertion works.

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