[GHC] #1991: Improve GADT type inference

[GHC]

#1991: Improve GADT type inference
-------------------------------------+--------------------------------------
 Reporter:  guest                    |          Owner:         
     Type:  feature request          |         Status:  closed 
 Priority:  normal                   |      Milestone:         
Component:  Compiler (Type checker)  |        Version:  6.6.1  
 Severity:  normal                   |     Resolution:  wontfix
 Keywords:                           |     Difficulty:  Unknown
 Testcase:                           |   Architecture:  Unknown
       Os:  Unknown                  |  
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Changes (by simonpj):

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

Comment:

 I'm afraid this is never going to compile without a type signature for
 `prIO`: in the presence of GADTs we lack principal types.  See e.g.
 [http://research.microsoft.com/%7Esimonpj/papers/gadt/index.htm Simple
 unification-based type inference for GADTs] for more info.

 The error message is a bit better in 6.8.
 {{{
 T1991.hs:11:6:
     GADT pattern match in non-rigid context for `NewRef'
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1991#comment:1>
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