Re: [Haskell-cafe] Closed Type Family Simplification

14 Aug 2014


      I think partially applied type synonyms (and type families) are
supposed to be disallowed, but I recently found out that since GHC
7.8, this is not the case anymore, but you get very weird other type
errors instead. I filed a bug [0], perhaps yours is a similar issue.

Erik

[0] https://ghc.haskell.org/trac/ghc/ticket/9433#ticket

On Thu, Aug 14, 2014 at 5:48 AM, Ian Milligan  wrote:
...
This is enough to demonstrate this error:
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE RankNTypes #-}
module TypeFamilyTest where
type family B f g a where B f g a = f (g a)
newtype C f a = C (f a)
t :: (forall a. a -> f a) -> (forall a. a -> g a) -> a -> C (B f g) a
t f g a = C (f (g a))
Is it not possible to use type families in this way?
On Wednesday, August 13, 2014 8:39:09 PM UTC-7, Ian Milligan wrote:
...
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  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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Re: [Haskell-cafe] Closed Type Family Simplification

Erik Hesselink