[GHC] #12542: Unexpected failure.. (bug?)

[GHC]

#12542: Unexpected failure.. (bug?)
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Ah now I see.  The offending where clause is equivalent to this:
 {{{
   where
     t = (viewVar, vewBnd)
     a = case t of (a,b) -> a
     b = case t of (a,b) -> b
 }}}
 What type does `t` get?
 {{{
    t :: forall d1 d2 a1 a2 b2. Binding d1, Binding d2)
                            => (d1 a1 -> Maybe Integer,
                                d2 (Fun a2 b2) -> Maybe Integer)
 }}}
 Now when we call `t` in the RHS of `a` we get the error message you see,
 because nothing fixes `d2` and hence we can't resolve `Binding d2` even
 though it is not in fact used.

 This doesn't arise when there is just one variable; try the same
 desugaring and you'll see.

 The monomorphism restriction means that we don't attempt to generalise
 over `Binding d1` etc, so none of this happens.

 I agree that this is perplexing, but I don't really know how to improve
 it.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12542#comment:5>
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