[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:  closed
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime           |            Test Case:
  performance bug                    |  perf/should_run/T15226, 15226a
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4796
       Wiki Page:                    |
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Comment (by dfeuer):

 Replying to [comment:16 simonpj]:
 > > seq# is intentionally lazy in its argument, to allow explicit ordering
 in an IO context
 >
 > Hmnm.  Can you give an example?  Nothing in `seq#`'s documentation says
 that.  It jolly well should!

 Considering `seq#` strict can be rather bad, I believe. If we turn `....
 seq# x s` into `case x of x' {DEFAULT__ -> .... seq# x' s}` then we'll see
 that `x'` is evaluated and erase the `seq#`. That sort of thing is the
 very sort of trouble `seq#` was intended to avoid.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15226#comment:19>
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