[GHC] #10769: Yet another crash from typed holes

[GHC]

#10769: Yet another crash from typed holes
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        Reporter:  rpglover64  |                   Owner:
            Type:  bug         |                  Status:  new
        Priority:  normal      |               Milestone:  7.10.3
       Component:  Compiler    |                 Version:  7.10.2
      Resolution:              |                Keywords:
Operating System:  Linux       |            Architecture:  x86_64 (amd64)
 Type of failure:  GHCi crash  |               Test Case:
      Blocked By:              |                Blocking:
 Related Tickets:              |  Differential Revisions:
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Comment (by bgamari):

 I would probably say that the bug was that GHC 7.10.2 didn't realize it
 needed a `Functor` instance (hence crashing at runtime as it had no
 dictionary to provide). This appears to be fixed in `master`.

 The problem here is that you program is ambiguous; GHC has no way of
 knowing which functor you mean here. GHC will gladly help you fill in the
 holes of an unambiguous program, but it won't make decisions on your
 behalf.

 This can be resolved by telling GHC which functor `a'` is supposed to be.
 For instance, perhaps you want `a` and `a'` to be values of the same
 functor,

 {{{#!hs
 {-# OPTIONS_GHC -fdefer-typed-holes #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 hylo :: Functor f => (a -> f a) -> (f b -> b) -> a -> b
 hylo a b = h where h = b . fmap h . a

 foo :: forall f a b. Functor f => (a -> f a) -> (f b -> b) -> a -> b
 foo a b = hylo a' b'
   where a' x = _
         b' :: f b -> b
         b' = _
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10769#comment:2>
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