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haskelltype-families

type families and deriving instances (Eq)

Is it possible to derive Eq instance for the B a, if given additional help, e.g. Eq a somewhere?

{-# LANGUAGE TypeFamilies #-}

class A a where
  type B a 
  somef :: a -> B a -> B a -> Bool

Question deriving instances with type families and instance definitions for type families are close. The following don't work nor similar modifications on type B a -line (or just tried the wrong ones). 

{-# LANGUAGE StandaloneDeriving #-}
-- deriving instance Eq (B a) -- illegal application
-- deriving instance Eq a => Eq (B a) -- illegal application

Constraining Eq a => A a didn't help. Adding constraint to somef compiles (somef :: Eq a => ...) and works for this method. However, in this case it would be nice to be able to tell that the type B a is equatable in general (such that non-equatable B a's would not be allowed) and not method-by-method.

Solution

  • I think this does the trick...

    {-# LANGUAGE TypeFamilies, FlexibleContexts #-}

class Eq (B a) => A a where
  type B a 
  somef :: a -> B a -> B a -> Bool

    As verification that it works, the following instance is accepted

    data HasEqInstance = HasEqInstance deriving Eq
instance A () where
  type B () = HasEqInstance
  somef = undefined

    But this one is rejected with No instance for (Eq NoEqInstance)

    data NoEqInstance = NoEqInstance
instance A () where
  type B () = NoEqInstance
  somef = undefined