[GHC] #13333: Typeable regression in GHC HEAD

24 Feb 2017

      #13333: Typeable regression in GHC HEAD
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I recently wrote some code while exploring a fix for #13327 that heavily
 uses poly-kinded `Typeable`. This compiles without issue on GHC 8.0.1 and
 8.0.2:

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module DataCast where

 import Data.Data
 import GHC.Exts (Constraint)

 data T (phantom :: k) = T

 data D (p :: k -> Constraint) (x :: j) where
   D :: forall (p :: k -> Constraint) (x :: k). p x => D p x

 class Possibly p x where
   possibly :: proxy1 p -> proxy2 x -> Maybe (D p x)

 dataCast1T :: forall k c t (phantom :: k).
               (Typeable k, Typeable t, Typeable phantom, Possibly Data
 phantom)
            => (forall d. Data d => c (t d))
            -> Maybe (c (T phantom))
 dataCast1T f = case possibly (Proxy :: Proxy Data) (Proxy :: Proxy
 phantom) of
                  Nothing -> Nothing
                  Just D  -> gcast1 f
 }}}

 But on GHC HEAD, it barfs this error:

 {{{
 Bug.hs:28:29: error:
     • Could not deduce (Typeable T) arising from a use of ‘gcast1’
         GHC can't yet do polykinded Typeable (T :: * -> *)
       from the context: (Typeable k, Typeable t, Typeable phantom,
                          Possibly Data phantom)
         bound by the type signature for:
                    dataCast1T :: (Typeable k, Typeable t, Typeable
 phantom,
                                   Possibly Data phantom) =>
                                  (forall d. Data d => c (t d)) -> Maybe (c
 (T phantom))
         at Bug.hs:(22,1)-(25,35)
       or from: (k ~ *, (phantom :: k) ~~ (x :: *), Data x)
         bound by a pattern with constructor:
                    D :: forall k (p :: k -> Constraint) (x :: k). p x => D
 p x,
                  in a case alternative
         at Bug.hs:28:23
     • In the expression: gcast1 f
       In a case alternative: Just D -> gcast1 f
       In the expression:
         case possibly (Proxy :: Proxy Data) (Proxy :: Proxy phantom) of
           Nothing -> Nothing
           Just D -> gcast1 f
    |
 28 |                  Just D  -> gcast1 f
    |                             ^^^^^^^^
 }}}

 That error message itself is hilariously strange, since GHC certainly has
 the power to do polykinded `Typeable` nowadays.

 Marking as high priority since this is a regression—feel free to lower the
 priority if you disagree.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13333
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