Re: [Haskell-cafe] Closed Type Family Simplification

13 Aug 2014

      I suspect the problem happens because I am trying to use a partially 
applied type family with a type constructor. However, in trying to 
replicate the problem I ran into another strange error message with the 
following program (using tagged and constraints as I was trying to keep it 
as close to the original program as possible)

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}

module TypeFamilyTest where

import GHC.Prim
import Data.Constraint
import Data.Tagged
import Data.Proxy

type family A :: * -> Constraint
type family B f g a where B f g a = f (g a)
newtype C f = C (forall a. Tagged a (A a :- A (f a)))

t :: forall f g a. C f -> C g -> C (B f g)
t (C x) (C y) = C z where
    z :: forall a. Tagged a (A a :- A (f (g a)))
    z = Tagged (trans (proxy x (Proxy :: Proxy (g a))) (proxy y (Proxy :: 
Proxy a)))

This fails with the error message

TypeFamilyTest.hs:21:19:
    Couldn't match type ‘a1’ with ‘g a1’
      ‘a1’ is a rigid type variable bound by
           a type expected by the context: Tagged a1 (A a1 :- A (f a1))
           at TypeFamilyTest.hs:21:17
    Expected type: Tagged a1 (A a1 :- A (f a1))
      Actual type: Tagged a1 (A a1 :- A (f (g a1)))
    Relevant bindings include
      z :: forall a. Tagged a (A a :- A (f (g a)))
        (bound at TypeFamilyTest.hs:23:5)
      y :: forall a. Tagged a (A a :- A (g a))
        (bound at TypeFamilyTest.hs:21:12)
      t :: C f -> C g -> C (B f g) (bound at TypeFamilyTest.hs:21:1)
    In the first argument of ‘C’, namely ‘z’
    In the expression: C z

for some reason C (B f g) expects Tagged a1 (A a1 :- A (f a1)) instead of 
Tagged a1 (A a1 :- A (f (g a1)))?

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 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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