Re: [GHC] #9838: PatternSynonyms and ConstraintKinds should not be required at the use site.

26 Nov 2014

      #9838: PatternSynonyms and ConstraintKinds should not be required at the use site.
-------------------------------------+-------------------------------------
              Reporter:  ekmett      |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.3
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by goldfire):

 Here's a test case for the !ConstraintKinds problem:

 {{{
 {-# LANGUAGE ConstraintKinds #-}

 module A where

 type EqShow a = (Eq a, Show a)
 }}}

 {{{
 module B where

 import A

 foo :: EqShow a => a -> String
 foo x = show x ++ show (x == x)
 }}}

 Compiling yields

 {{{
 B.hs:5:8:
     Illegal tuple constraint: EqShow a
       (Use ConstraintKinds to permit this)
     In the type signature for ‘foo’: foo :: EqShow a => a -> String
 }}}

 I'd love to see this fix get in for 7.10, but I won't have a chance to
 work on this before the (new) freeze date on Friday.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9838#comment:4
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