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

I don't have GHC 7.0.3 available, but I think this should work with it.

You could pass the dictionaries around manually like this (using Context = Show as an example):

{-# LANGUAGE ScopedTypeVariables, TypeFamilies, ExistentialQuantification #-}

data ShowDict a = Show a => ShowDict

class Class a where
  data Associated a :: * -> *

  getShow :: ShowDict (Associated a b)

-- Convenience function
getShowFor :: Class a => Associated a b -> ShowDict (Associated a b)
getShowFor _ = getShow

showAssociated :: Class a => Associated a b -> String
showAssociated a = 
  case getShowFor a of
    ShowDict -> -- Show (Associated a b) is made available by this pattern match 
      show a

instance Class Int where
  data Associated Int b = Foo deriving Show

  getShow = ShowDict

main = print $ showAssociated Foo

This is somewhat similar to the function copying you propose, but advantages are:

• Avoids repetition (of `Context`'s method signatures)
• Having `Show Baz` in context is somewhat more powerful than just having a function for showing a `Baz`, since it allows you to call (library) functions which require `Show Baz`, or use implied instances like `Show [Baz]`:

showAssociateds :: forall a b. Class a => [Associated a b] -> String
showAssociateds as = 
  case getShow :: ShowDict (Associated a b) of
    ShowDict ->
      show as

The main disadvantage is that using getShow always requires an explicit type signature (functions like getShowFor can mitigate this).