[GHC] #10072: Panic: generalised wildcards in RULES

[GHC]

#10072: Panic: generalised wildcards in RULES
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        Reporter:  thomasw           |                   Owner:  thomasw
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.1-rc2
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  Compile-time      |               Test Case:
  crash                              |                Blocking:
      Blocked By:                    |  Differential Revisions:
 Related Tickets:                    |
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Comment (by thomasw):

 Replying to [comment:2 simonpj]:
 > We should never generalise over wildcards!

 I'm confused, we do generalise over wildcards when possible, right? When I
 say "the wildcard is generalised over", I mean that when the wildcard is
 not instantiated to a monotype, we replace it with a fresh type variable
 and universally quantify it. This is what we have being doing the whole
 time, e.g.:

 {{{#!hs
 foo :: _ -> _
 foo x = x
 -- Inferred: w_ -> w_
 }}}


 What I expected in the example in the description above (and what indeed
 happens) is that the wildcard is generalised over and that we get `a ->
 w_` as type for `f`.


 > Thomas, are you ok to look into this one?  Yell if you need help.

 I see two ways to solve it:

 1. Don't print skolem information when there is none (easy).
 2. Generate an `Implication` with the right skolem information (harder).

 Which solution do you prefer? If you prefer 2, I'd appreciate some
 pointers as to where/when these `Implication` constraints should be
 generated.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10072#comment:3>
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