[GHC] #7071: Refactoring arrows

[GHC]

#7071: Refactoring arrows
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    Reporter:  simonpj           |       Owner:                  
        Type:  bug               |      Status:  new             
    Priority:  normal            |   Milestone:                  
   Component:  Compiler          |     Version:  7.4.2           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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 At the moment arrow commands re-use `HsExpr` for Proc expressions.  But
 with Dan Winograd-Cort I concluded that the best thing by far would be to
 separate the data type of HsExpr from that of arrow commands. I think that
 would lead to a substantial tidy up.

 There is also a nasty lurking bug in the type checking of commands; see
 line 290 of TcArrows. Here we call the unifier, but do not do anything
 with the coercion it returns.  This is plainly wrong and will bite
 eventually. But I don't understand this code well enough to fix it.

 In short, I am not confident of the arrows implementation at the moment.

 Several tickets are blocked on this
  * #5022 (which is marked as fixed but is a pretty tricky case)
  * #5267
  * #5777
  * #4359 proposal for `proc case` and multi-branch if

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7071>
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