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

You can write this:

class (SetLike (EfficientSet a), Elem (EfficientSet a) ~ a) =>
      HasEfficientSet a where
    type EfficientSet a

If you associate the Elem type family with the SetLike class, you probably wouldn't even need the SetLike superclass constraint:

class SetLike s where
    type Elem s
    insert :: Elem s -> s -> s
    member :: Elem s -> s -> Bool

class Elem (EfficientSet a) ~ a => HasEfficientSet a where
    type EfficientSet a