[GHC] #13933: Support Typeable instances for types with coercions

7 Jul 2017

      #13933: Support Typeable instances for types with coercions
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           Reporter:  goldfire       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.3
           Keywords:  TypeInType,    |  Operating System:  Unknown/Multiple
  Typeable                           |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 If I say

 {{{#!hs
 {-# LANGUAGE GADTs, TypeApplications, TypeInType #-}

 module Bug where

 import Type.Reflection

 data G a where
   MkG :: a ~ Bool => G a

 rep = typeRep @MkG
 }}}

 I get

 {{{
 Bug.hs:10:7: error:
     • No instance for (Typeable <>) arising from a use of ‘typeRep’
     • In the expression: typeRep @MkG
       In an equation for ‘rep’: rep = typeRep @MkG
    |
 10 | rep = typeRep @MkG
    |
 }}}

 First off, the error message is confusing, mentioning the mysterious `<>`.
 But more importantly, it would be nice if the `Typeable` mechanism
 supported coercions.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13933
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