[GHC] #10662: GHC warning shows technical summary of AST instead of the user's code

[GHC]

#10662: GHC warning shows technical summary of AST instead of the user's code
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        Reporter:  kolmodin          |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.10.2
       Component:  Compiler          |                 Version:  7.10.2-rc2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by simonpj):

 Quite right!

 Reason: this message is generated by the desugarer, which only has
 typechecked code to print.

 Solution: when printing typechecked code, suppress details generated by
 the type checker itself.  Details:

  * Sometimes we want to see those details, certainly during compiler
 debugging. And just conceivably for savvy users.  So we need a flag to
 control it.

  * Quite a lot of supression goes on already. Example: `pprHsWrapper` uses
 `-dppr-debug` to control whether a `HsWrapper` get printed.  But the
 `-dppr-debug` flag is a pretty low level thing that controls a lot of
 other debug-printing stuff too.

  * Nowadays we have `-fprint-explicit-foralls` and `-fprint-explicit-
 kinds`, so it might be better to add a new flat `-fprint-evidence` (or
 something) which shows you the extra evidence generated by the type
 checker.

 Nothing hard here, if someone would like to take it on.  I can advise.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10662#comment:1>
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