[Haskell-cafe] Closed Type Family Simplification

[Ian Milligan]

Here is a small example which shows the problem

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE AllowAmbiguousTypes #-}

module TypeFamilyTest where

import GHC.Prim

type family A ∷ * → Constraint

type family C f g a where C f g a = f (g a)

a ∷ A (f (g a)) ⇒ ()
a = ()

b ∷ A (C f g a) ⇒ ()
b = a


On Wednesday, August 13, 2014 6:54:45 PM UTC-7, Richard Eisenberg wrote:
>
> Your operating assumption sounds right. Do you have a complete, minimal 
> example showing the error? If not, I recommend using -fprint-explicit-kinds 
> to see if kinds are getting in your way at all. 
>
> Richard 
>
> On Aug 13, 2014, at 8:02 PM, Ian Milligan <ianm... at gmail.com <javascript:>> 
> wrote: 
>
> > When a closed type family has only one instance it seems like it should 
> never fail to simplify. Yet this doesn't appear to be the case. When I 
> defined (in GHC 7.8.3) the closed type family 
> > type family (:.:) f g a where (:.:) f g a = f (g a) 
> > I get errors such as 
> > 'Could not deduce (Object c3 ((:.:) f g a) ~ Object c3 (f (g a)))' 
> > (where Object is a Constraint family), indicating that f (g a) is not 
> being substituted for (:.:) f g a as desired. Any idea why this happens? 
> > _______________________________________________ 
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>
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