[GHC] #9495: Do What I Mean RULES for foldr2 look shady

[GHC]

#9495: Do What I Mean RULES for foldr2 look shady
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              Reporter:  dfeuer      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  highest     |        Milestone:  7.8.4
             Component:              |          Version:  7.8.3
  libraries/base                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  Runtime     |  Related Tickets:
  crash                              |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 To broaden this a bit, it is worth remembering that ''the main
 `foldr/build` rule itself is unsound in the presence of `seq`''.  See, for
 example, the
 [http://www.haskell.org/haskellwiki/Correctness_of_short_cut_fusion#In_the_presence_of_seq
 Haskell wiki page].

 This is not nice.  The "right" solution is for `seq` to be an operation of
 a type class, something that was the case in Haskell originally, and then
 changed after ''extensive'' debate on the Haskell committee.

 So currently we are stuck in the unsatisfactory situation that certain
 optimisations, which have a generally very beneficial effect on
 performance, can change termination behaviour. Eta reduction/expansion is
 another.

 In that general context I don't have a strong opinion about the `foldr2`
 question.  I'd consult the Core Libraries Committee.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9495#comment:6>
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