[GHC] #10097: GHC 7.11 errors on dictionary casting tricks

[GHC]

#10097: GHC 7.11 errors on dictionary casting tricks
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        Reporter:  heisenbug         |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by goldfire):

 I don't like that there's been a typechecker regression here, but this
 doesn't seem like an outright bug to me. The one bit I'm quite unsure of
 is the eta-contraction... and my reasoning-about-higher-rank-eta-
 contracted-functions neuron seems to be enjoying the snow day here. So
 I'll eta-expand:

 {{{
 iFeelDirty x = unsafeCoerce x
 }}}

 Now, I can see why your error appears. When you say `x`, GHC will try to
 instantiate the implicit parameters of `x`, namely `ts` and `List f ts`.
 But, GHC has no guidance as to what these should be, because it can't
 glean anything useful from `unsafeCoerce`. Some more type annotations
 might be helpful, as might be newtype wrappers to accurately control what
 you're `unsafeCoerce`ing.

 So, I have two conclusions:
 * I'm happy enough with an error in the eta-expanded version, but I agree
 that the eta-contracted one should probably work.

 * I don't think this bug shows up a problem with the dirty dictionary-
 casting trick. That should still be valid. It's just a typechecker
 behavior change, which (I believe) could be overcome with more types.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10097#comment:1>
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