[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):

 It does seem to make sense to handle the scope issues in the renamer, but
 I think there's another issue here beyond holey scopes.  Here's the type
 rule for ArrForm, just in the unary case (fixing a typo, which wouldn't
 help):
 {{{
      G      |-b  c : [s1 .. sm] s
      pop(G) |-   e : forall w. b ((w,s1) .. sm) s -> a ((w,t1) .. tn) t
      w \not\in (s, s1..sm, t, t1..tn)
      ----------------------------------------------
      G |-a  (| e c |)  :  [t1 .. tn] t
 }}}
 So e is checked in the outer scope, but you also need to be able to
 generalize over the type variable w, so it can't occur free in pop(G)
 (which it does in the first example above).  This is part of ensuring that
 operators like e are oblivious to the way we plumb the environment through
 the arrows.  The rest is the constraint on w in the above rule.  The si
 and ti are checked by the fragment you mention above.  The check on t
 seems to have disappeared in 7.0, as seen in the second example I gave.
 It seems the check on s was never implemented: the following is wrongly
 accepted even by 6.12:
 {{{
 should_fail_too x = proc y -> (| bad_op (returnA -< x) |)
   where
     bad_op :: Arrow a => a env env -> a env Bool
     bad_op _ = arr (const True)
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5267#comment:11>
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