Wildcards in type functions

[MartijnVanSteenbergen]

I would like to be able to use wildcards in type synonym family instances, so that I can write:

type instance ErrorAlg (f :>: _) e a = ErrorAlg f e a

[simonpj]

Very reasonable idea. I've added it to our list of type-function feature requests at  http://hackage.haskell.org/trac/ghc/wiki/TypeFunctionsStatus

Simon

[MartijnVanSteenbergen]

Continuing this line of thought, instances for this family go hand in hand with instances for this type class:

class MkErrorAlgebra f where
  mkErrorAlgebra :: ErrorAlg f e a -> ErrorAlgPF f e a

Would it then also make sense to write:

instance MkErrorAlgebra f => MkErrorAlgebra (f :>: _) where
  mkErrorAlgebra alg         (Tag f)           = mkErrorAlgebra alg f

?

Just a wild idea which I'm not sure yet I like or dislike. It does fit in with the idea of not making up names if you don't have to. For that reason I also prefer writing data D :: <kind> where for GADTs rather than making up arbitrary names for the type arguments. Going even further, this idea can be extended to e.g. data Const a _ = Const a for normal ADTs.

Okay, enough wild ideas for now. :-)