[GHC] #15226: GHC doesn't know that seq# produces something in WHNF

[GHC]

#15226: GHC doesn't know that seq# produces something in WHNF
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4796
       Wiki Page:                    |
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Comment (by dfeuer):

 I'm also not sure it's worth the trouble, although the current state of
 affairs seems a bit tricky to document in the Haddocks for `evaluate`. But
 considering `Control. Parallel.Strategies.rdeepseq`, I realized that even
 this binder swap, in combination with what I've already done, isn't really
 quite enough. Suppose we have

 {{{#!hs
 let e = x + 3 :: Int
 in case seq# e s of
      (# s', e' #) -> E
 }}}

 We'd actually like to know that not only `e` and `e'`, but also `x`, are
 evaluated in `E`, because `e` is strict in `x`. So if we do a binder swap,
 we should do it for all the variables the scrutinee is strict in that are
 not already known to be evaluated.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15226#comment:12>
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