[GHC] #3085: warn about language extensions that are not used

[GHC]

#3085: warn about language extensions that are not used
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              Reporter:              |            Owner:
  PVerswyvelen                       |           Status:  new
                  Type:  feature     |        Milestone:  7.10.1
  request                            |          Version:  6.10.1
              Priority:  normal      |         Keywords:  warnings
             Component:  Compiler    |  extensions language
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:  #9757
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by Lemming):

 Replying to [comment:33 thomie]:
 > To cite rwbarton from #9757:
 > > I think the purpose of compiler warnings should be to warn about code
 that is probably bad, not code that is probably fine that could be written
 in a (potentially) better way. (After all, we don't warn about excess
 parentheses in code either.) That is the domain of hlint, as Richard
 notes.
 >
 > Anyone disagree with a `wontfix` here as well?

 GHC already warns about unused variables and unused imports. Is the
 warning about unused instances much different? If you think, that the
 warning is not essential it may not be enabled by -Wall. And there are
 already non-Wall warnings that pointed me to bad code, like the warning
 about incomplete case analysis of pattern matches in lambda expressions.
 (I think it's `-fwarn-incomplete-uni-patterns`.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3085#comment:34>
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