[GHC] #8997: Warn about unused parameters in recursive definitions

[GHC]

#8997: Warn about unused parameters in recursive definitions
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       Reporter:  nomeata           |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  low               |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 I’m correcting student’s exams right now; a good source of bad programs
 :-)

 Consider this program:
 {{{
 foo _ y [] = y
 foo z y (_:xs) = foo z y xs
 }}}

 It would be nice if GHC would warn that the first parameter of `foo` is
 not used. IIRC GHC warns about unused constructors even when they occur on
 RHSs.

 If `foo` is passed to some higher order function that expects a particular
 type, removing the parameter would require a wrapper for there, or at
 least an anonymous lambda... but it’s probably still worth notifying the
 programmer, as more likely there is a bug or misunderstanding in such
 code.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8997>
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