[GHC] #5267: Arrow command combinators

[GHC]

#5267: Arrow command combinators
----------------------------------------+-----------------------------------
  Reporter:  peteg                      |          Owner:                  
      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  |  
----------------------------------------+-----------------------------------

Comment(by ross):

 > Bug: moving ite_perm out of the where block works.

 This one:
 {{{
 ite' cA tA eA = proc x ->
   do c <- cA -< x
      (| ite_perm (returnA -< c) |)
   where ite_perm i = ite i tA eA
 }}}
 works for me in 7.0.1, 7.0.4 and HEAD, but not 6.12.2.

 > Bug: we can't write the type signature we want.

 {{{
 ite' :: ArrowChoice (~>) => A (~>) env Bool -> A (~>) env d -> A (~>) env
 d -> A (~>) env d
 ite' cA tA eA = proc x ->
   do c <- cA -< x
      (| (ite_perm tA eA) (returnA -< c) |)

 ite_perm :: ArrowChoice (~>) => A (~>) env d -> A (~>) env d -> (env ~>
 Bool) -> A (~>) env d
 ite_perm tA eA i = ite i tA eA
 }}}
 We have {{{ite_perm tA eA :: (env ~> Bool) -> A (~>) env d}}}, and the
 compiler can't generalize {{{env}}} as required by the banana brackets,
 because it occurs in the type environment.  So it's following the rules;
 the question is whether the rules can be safely relaxed.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5267#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler