Is it possible to introduce additional type variables into a superclass-constraint?

Question

When dealing with type families, it is often handy to use equality constraints to avoid having to repeat some type-function\'s name in a signature:

class Foo         


        

Answer 1

It surprised me to learn you can't do that (I've used the same technique and know it works in instance declarations), but there seems to be a long-standing GHC feature request to support this.

Maybe you can use ConstraintKinds to get the same benefit:

{-# LANGUAGE TypeFamilies , FlexibleContexts , ConstraintKinds #-}
import Data.Monoid

class Foo f where
  type BulkyAssociatedType f :: *

type B f = (Monoid (BulkyAssociatedType f))

class ( Foo f, B f) => Bar f