Re: [GHC] #8581: Add support for explicitly-bidirectional pattern synonyms

22 Aug 2014

      #8581: Add support for explicitly-bidirectional pattern synonyms
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              Reporter:  cactus      |            Owner:  cactus
                  Type:  feature     |           Status:  closed
  request                            |        Milestone:  7.10.1
              Priority:  normal      |          Version:
             Component:  Compiler    |         Keywords:
            Resolution:  fixed       |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:  5144
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by mpickering):

 That is exactly what I mean, thank you for taking the time to clarify
 my comment. To once again be more specific, the code example I posted
 fails to type check with the following error message.

 {{{

 fix.hs:27:14:
     Could not deduce (Construct a) arising from a use of ‘construct’
     from the context (View a)
       bound by the type signature for Main.$WAdd :: View a => a -> a -> a
       at fix.hs:1:1
     Possible fix:
       add (Construct a) to the context of
         the type signature for Main.$WAdd :: View a => a -> a -> a
     In the expression: (construct (AddF a b))
     In an equation for ‘$WAdd’: $WAdd a b = (construct (AddF a b))

 fix.hs:30:12:
     Could not deduce (Construct a) arising from a use of ‘construct’
     from the context (View a)
       bound by the type signature for Main.$WNum :: View a => Int -> a
       at fix.hs:1:1
     Possible fix:
       add (Construct a) to the context of
         the type signature for Main.$WNum :: View a => Int -> a
     In the expression: (construct (NumF n))
     In an equation for ‘$WNum’: $WNum n = (construct (NumF n))

 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8581#comment:11
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