[GHC] #13465: Foldable deriving treatment of tuples is too surprising

21 Mar 2017

      #13465: Foldable deriving treatment of tuples is too surprising
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           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{
 *> :set -XDeriveFoldable
 *> data Foo a = Foo ((a,a),a) deriving Foldable
 *> length ((1,1),1)
 1
 *> length $ Foo ((1,1),1)
 3
 }}}

 I think the right thing is probably to refuse to derive `Foldable` if
 components of a tuple other than the last one may contain the type
 variable we're looking for.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13465
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