Re: [Haskell-cafe] Type synonym family inside type class

Hi Sean, Sean Leather wrote:

Perhaps this isn't answering your question, but you can turn the above into an associated type as follows.

class MkErrorAlg f where type ErrorAlg (f :: (* -> *) -> * -> *) :: * -> * -> * mkErrorAlg :: ErrorAlg f e a -> f (K0 a) ix -> Either e a

That is true. But how would you translate the following instance to the new version?

type instance ErrorAlg (K b :*: f) e a = b -> ErrorAlg f e a

I don't know how to do that without introducing a new newtype that I can partially parametrize. I don't want to have to introduce a newtype for that, especially because it defeats the purpose of (in this specific case) generically deriving a user-friendly type. Groetjes, Martijn.