[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 accepts invalid program  |  
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Comment(by ross):

 Here are some examples from our desugaring discussion, which may or may
 not help.
 {{{
 add :: Arrow a => a b Int -> a b Int -> a b Int
 add f g = proc z ->
         (f -< z) `bind` \ x ->
         (g -< z) `bind` \ y ->
         returnA -< x+y

 bind :: Arrow a => a e b -> a (e, b) c -> a e c
 u `bind` f = arr Prelude.id &&& u >>> f
 }}}
 Here we have infix `ArrForm`s with `bind` as the function.
 {{{
 ite :: ArrowChoice a => a env Bool -> a env d -> a env d -> a env d
 ite iA tA eA = proc env ->
         (iA -< env) `bind` \ i ->
         (| ifThenElseA (tA -< env) (eA -< env) |) i

 ifThenElseA :: ArrowChoice a => a e r -> a e r -> a (e, Bool) r
 ifThenElseA thenPart elsePart = arr split >>> thenPart ||| elsePart
   where
         split (e, True) = Left e
         split (e, False) = Right e
 }}}
 In these cases one can see the arity from the code, but one can also eta-
 contract the above to
 {{{
 ite' :: ArrowChoice a => a env Bool -> a env d -> a env d -> a env d
 ite' iA tA eA = proc env ->
         (iA -< env) `bind` (| ifThenElseA (tA -< env) (eA -< env) |)
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5267#comment:15>
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