[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:  #1537,
                                              |  #3613
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Comment (by simonpj):

 The VOODOO CODING is of course wrong.  Take a look at the version for
 `BindStmt` in (for example) `tcMcStmt`, in `TcMatches` line 511.

 We are typechecking `pat <- rhs; body` as if it was `(>>=) rhs (\x ->
 body)`.  So we instantiate `(>>=)`, expecting its type to have the shape
 {{{
     rhs_ty -> (pat_ty -> body_ty)
 }}}
 That's what `tcSyntaxOp` does.

 Then we check that `pat` really does have type `pat_ty` and `rhs` really
 does have type `rhs_ty`.

 Presumably you need to do similar things for arrows, but I'm not deep
 enough into it to know how.  Doing the instantiation once, at `CmdTop`
 will almost certainly not work.

 I'm sorry not to be more helpful.

 Why not follow Ross's suggestions in comment 17?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7828#comment:26>
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