[GHC] #9111: base should export Typeable instances of its promoted data constructors

14 May 2014

      #9111: base should export Typeable instances of its promoted data constructors
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       Reporter:  goldfire          |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 As initially reported in #8486, `base` does not export `Typeable`
 instances for its promoted data constructors, as witnessed by this:

 {{{
 {-# LANGUAGE DataKinds #-}

 import Data.Typeable

 x = typeRep (Proxy :: Proxy True)
 }}}

 yields

 {{{
     No instance for (Typeable 'True) arising from a use of ‘typeRep’
     In the expression: typeRep (Proxy :: Proxy True)
     In an equation for ‘x’: x = typeRep (Proxy :: Proxy True)
 }}}

 It is indeed possible to derive these instances locally with standalone
 deriving, but that seems very problematic, should multiple modules do that
 independently.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9111
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