[GHC] #10958: "Annotating pure code for parallelism" docs based on old par/pseq primitives

[GHC]

#10958: "Annotating pure code for parallelism" docs based on old par/pseq
primitives
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           Reporter:  robstewartuk   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  lowest         |         Milestone:
          Component:  Documentation  |           Version:  7.10.2
           Keywords:  parallelim     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The example code in GHC 7.10.2 docs "7.28. Concurrent and Parallel
 Haskell", then in "7.15.4. Annotating pure code for parallelism" the
 recommendation is to use `par` and `pseq`. These primitives pre-dates the
 strategy combinators `rpar` and `rseq`. The example given is:

 {{{#!hs
 import Control.Parallel

 nfib :: Int -> Int
 nfib n | n <= 1 = 1
        | otherwise = par n1 (seq n2 (n1 + n2 + 1))
                      where n1 = nfib (n-1)
                            n2 = nfib (n-2)
 }}}

 The paper "Seq no more: Better Strategies for Parallel Haskell"
 ([http://www.macs.hw.ac.uk/~hwloidl/publications/strategies10.pdf])
 advocates `Eval` as an "evaluation order" monad, as preferable (versus
 `par` and `seq`) for loose control of evaluation order of parallelism.
 Moreover, Simon Marlow's "Parallel and Concurrent Programming in Haskell"
 doesn't mention the `par` and `seq` primitives at all.

 Should the GHC docs for "Concurrent and Parallel Haskell" encourage the
 use Eval combinators, either in addition to or instead of the `par` and
 `seq` primitives? I'd be happy to contribute an update to the docs if
 people agree to a shift in emphasis towards the Eval monad in the docs.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10958>
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