[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):

 I wrote


 > 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`.

 The last sentence is terribly important!  We are not just checking that
 `(>>=)` has a type of the right shape (which is, I gather from your
 question, what you think is going on), ''but also that it's type can be
 instantiated to match the particular pattern `pat` and the particular rhs
 `rhs`''.   You aren't doing that with `(>>>)` which is why you have all
 those uninstantiated type variables.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7828#comment:29>
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