[GHC] #10985: When a "non-exhaustive pattern"-error occurs, output the arguments (if possible)

[GHC]

#10985: When a "non-exhaustive pattern"-error occurs, output the arguments (if
possible)
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        Reporter:  Watercrystal      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
  (Debugging)                        |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10972            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I like this idea. In certain easy cases (pattern-matching against a
 monomorphic type that has an obvious `Show` instance) this would be quite
 easy to add, I think. Harder cases would be, well, harder. But perhaps the
 easy case is enough.

 Note that it would be hard to do a check to see if a type variable is
 instantiated to a type that has a `Show` instance.

 Or, instead of using `Show`, we could use whatever GHCi's `:force` command
 uses to print a value. (This doesn't require a `Show` instance.) It's even
 conceivable to use a `Show` instance if one is obviously available and
 fallback to the `:force` thing otherwise.

 Any volunteers to concretely specify this behavior and implement?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10985#comment:1>
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