[GHC] #8704: Use GHC.Exts.build in randoms, randomRs to achieve fusion

[GHC]

#8704: Use GHC.Exts.build in randoms, randomRs to achieve fusion
-------------------------------------+------------------------------------
        Reporter:  ion1              |            Owner:
            Type:  feature request   |           Status:  patch
        Priority:  normal            |        Milestone:
       Component:  libraries/random  |          Version:  7.6.3
      Resolution:                    |         Keywords:  fusion
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #4218
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Comment (by ion1):

 Yes, I tested with

 {{{
 main = do { gen <- newStdGen; mapM_ print (randoms gen) }
 }}}

 which resulted in the list data structure being optimized away entirely
 (along with the pattern match for the empty case in `mapM_`).

 I got around to doing some benchmarking after posting the patch and I seem
 unable to come up with code for which `randoms` fusion makes a difference
 in performance, though. Unless someone else thinks of something I didn’t,
 this might not be worth it.

 The `seq` fix for part of #4218 can be done independently.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8704#comment:3>
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