Re: [GHC] #13006: Possible program should type check but does not using Implicit Parameters and Vectors

19 Dec 2016

      #13006: Possible program should type check but does not using Implicit Parameters
and Vectors
-------------------------------------+-------------------------------------
        Reporter:  clinton           |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2-rc2
      Resolution:  invalid           |             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:                    |
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Comment (by clinton):

 @simonpj: I'm not sure if I should put in a separate bug report or if I'm
 completely confused, but when I compile the following:
 {{{#!hs
 {-# LANGUAGE TypeFamilyDependencies #-}

 type family F x = t | t -> x

 myId :: (F a ~ F b) => a -> b
 myId = id

 main = pure ()
 }}}

 I get the error:

 {{{
 Could not deduce: a ~ b
 from the context: F a ~ F b
 }}}

 Isn't this what ticket #10833 discusses, namely that GHC does not derive
 `a ~ b` from `F a ~ F b`?

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13006#comment:6
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