[GHC] #12441: Conflicting definitions error does not print explicit quantifiers when necessary

[GHC]

#12441: Conflicting definitions error does not print explicit quantifiers when
necessary
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        Reporter:  ezyang            |                Owner:  philderbeast
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2734
       Wiki Page:                    |
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Comment (by simonpj):

 Thanks for doing this patch.

 To me the extra-code to extra-benefit ratio feels a bit low.  My
 suggestion: always print foralls when displaying the results from hi-boot-
 file mis-matches.

 * It's a relatively corner case that few people encounter, and those that
 do are probably not going to be puzzled by a forall.
 * If we start getting complaints that the foralls are cluttering things up
 for someone we can consider elaborating.

 In short, let's not solve a problem we don't yet have.

 Even with the "always print foralls" idea, the whole ppr-iface stuff is
 getting a bit baroque.  We have

 * The print-foralls flag
 * The `ShowSub` info
 * Sometimes `hdr_only` flag goo

 Could we combine all three into a single record with three fields?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12441#comment:6>
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