Type constraints on all type family instances

Question

I suppose what I want is impossible without Template Haskell but I\'ll ask anyway.

I have an interface for types like Data.Set and Data.IntSet

Answer 1

Using GHC 7.4's constraint kinds you could have something like

type EfficientSetLike a = (SetLike (EfficientSet a),Elem (EfficientSet a) ~ a)

You can (with appropriate extensions) get constraints like this in earlier versions of GHC

class (SetLike (EfficientSet a),Elem (EfficientSet a) ~ a) => EfficientSetLike a 
instance (SetLike (EfficientSet a),Elem (EfficientSet a) ~ a) => EfficientSetLike a 

But, the new style type declaration is much nicer.

I'm not exactly sure what you are looking for, but it sounds like you just want easier to write/understand constraint signatures, in which case this will work.