[GHC] #7828: RebindableSyntax and Arrow

[GHC]

#7828: RebindableSyntax and Arrow
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        Reporter:  AlessandroVermeulen        |            Owner:
            Type:  bug                        |  jstolarek
        Priority:  normal                     |           Status:  new
       Component:  Compiler (Type checker)    |        Milestone:  7.10.1
      Resolution:                             |          Version:  7.6.2
Operating System:  Unknown/Multiple           |         Keywords:
 Type of failure:  GHC rejects valid program  |     Architecture:
       Test Case:                             |  Unknown/Multiple
        Blocking:                             |       Difficulty:  Unknown
                                              |       Blocked By:
                                              |  Related Tickets:
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Comment (by simonpj):

 Yes, I think if we made the change suggested here, arrow notation would
 become rebindable.

 I think the idea would be that each construct becomes decorated with the
 overloaded operations necessary to desugar it.  Indeed, in the case of
 monad syntax, the effect is exactly as if you first desugared and then
 typechecked.  For example `x <- e; rest` desugars into something like `e
 >>= \x -> rest`.  So to typecheck `x <- e` (a `BindStmt`), we look up
 `>>=`, instantiate its type, unify `e`'s type with the first argument type
 of `>>=`, and so on.  We hang onto the instantiated `>>=` in the
 typechecked `BindStmt`.

 The idea is to do the same for arrows, but I have little idea of how hard
 it is.  But the thought experiment of first desugaring and then type
 checking may be an instructive one.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7828#comment:8>
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