How to put constraints on the associated data?

Question

I would like to state that the associated data is always an instance of a certain class.

class (Context (Associated a b)) => Class a where
  data Associated a         


        

Answer 1

As was pointed out by @SjoerdVisscher, using forall on the left side of => in a class or instance is actually not ok, at least not yet, though my specific example does work in ghc-7.4.

---

This way it seems to work:

{-# LANGUAGE FlexibleInstances    #-}
{-# LANGUAGE TypeFamilies         #-}
{-# LANGUAGE Rank2Types           #-}
{-# LANGUAGE ConstraintKinds      #-}
{-# LANGUAGE UndecidableInstances #-}

class Context c where
  func1 :: c -> String

class (forall b. Context (Associated a b)) => Class a where
  data Associated a :: * -> *

newtype ClassTest = ClassTest { runClassTest :: String }

instance (forall b. Context (Associated ClassTest b)) => Class ClassTest where
  data Associated ClassTest b = ClassTestAssoc b (b -> ClassTest)

instance Context (Associated ClassTest b) where
  func1 (ClassTestAssoc b strFunc) = runClassTest $ strFunc b

main = putStr . func1 $ ClassTestAssoc 37 (ClassTest . show)

The extra forall b constraint in the instance seems a bit ugly and redundant, but apparently it's necessary.

$ runghc-7.4.1 tFamConstraint0.hs
37