[GHC] #13705: Failure of improvement for type-family dependencies

16 May 2017

      #13705: Failure of improvement for type-family dependencies
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider
 {{{
 {-# LANGUAGE TypeFamilyDependencies #-}

 data D x

 type family F t = s | s -> t
 type instance F (D t) = D (F t)

 f :: F s -> ()
 f _ = ()

 g :: D (F t) -> ()
 g x = f x
 }}}
 which was presented in [https://mail.haskell.org/pipermail/haskell-
 cafe/2016-October/125375.html this email] from Clinton Mead.

 It yields
 {{{
 Tx.hs:14:9: error:
     • Couldn't match expected type ‘F s0’ with actual type ‘D (F t)’
       The type variable ‘s0’ is ambiguous
 }}}
 But it should work fine

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13705
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