[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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Changes (by goldfire):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 This seems to be exactly the scenario in #6075. Essentially, GHC checks
 patterns from the outside working in, so it sees the type before the
 constructor, and before it knows about the GADT pattern match. I believe
 I've worked around this by doing the following painful construction:

 {{{
 lengthSing = case sing :: Sing xs of
   SNil -> Tagged 0
   SCons -> case sing :: Sing xs of
     (SCons :: Sing (x ': xs')) -> Tagged (1 + untag (lengthSing :: Tagged
 xs' Int))
 }}}

 There may be a better way, but that's what I've done and it works.

 I suppose you could re-open as a feature request to change this behavior,
 if you feel so inclined.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9108#comment:1>
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