[GHC] #9108: GADTs and pattern type annotations?

[GHC]

#9108: GADTs and pattern type annotations?
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        Reporter:  edsko             |            Owner:
            Type:  bug               |           Status:  closed
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:  invalid           |         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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Comment (by edsko):

 Hmmmm, nasty :( I worked out in a different way, and Andres suggested yet
 another way (using a type family), but it seems me me that this evaluated-
 before-pattern-match behaviour is unfortunate behaviour; mightn't have
 made a difference before GADTs but with GADTs is just seems the wrong way
 around? Are there reasons ''not'' to evaluate the pattern match before the
 type annotation? (It's a tricky thing, type checking for GADTs, I realize
 that :)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9108#comment:2>
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