[GHC] #5267: Arrow command combinators

[GHC]

#5267: Arrow command combinators
----------------------------------------+-----------------------------------
  Reporter:  peteg                      |          Owner:  ross            
      Type:  bug                        |         Status:  new             
  Priority:  normal                     |      Milestone:  7.4.1           
 Component:  Compiler                   |        Version:  7.0.3           
Resolution:                             |       Keywords:                  
  Testcase:                             |      Blockedby:                  
Difficulty:                             |             Os:  Unknown/Multiple
  Blocking:                             |   Architecture:  Unknown/Multiple
   Failure:  GHC rejects valid program  |  
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Comment(by ross):

 Here's a simpler snippet illustrating the same bug:
 {{{
 should_fail v b = proc x -> (| f (returnA -< v) |)
   where f a = a <+> b
 }}}
 The deleted test rightly rejected this because {{{f :: (env ~> r) -> (env
 ~> r)}}} and the type variable {{{env}}} occurs in {{{b :: env ~> r}}}.

 Another thing that the deleted test prevented was
 {{{
 should_also_fail x = proc y -> (| bad_op (returnA -< x) |)
   where
     bad_op :: Arrow a => a env b -> a env env
     bad_op = const returnA
 }}}
 where the type variable {{{env}}} occurs in the output type of the arrow
 constructed by {{{bad_op}}}.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5267#comment:9>
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