[Haskell-cafe] par / pseq example
waldmann at imn.htwk-leipzig.de
Wed Dec 15 11:30:32 CET 2010
I was trying to find an easy example for par/pseq,
and I finally used plain mergesort ( on Prelude. )
and I observed that it is best to give +RTS -A1G (or more)
such that there are only very few garbage collections.
This roughly cuts execution time in half
(from +RTS -N1 to -N4, on an i7 CPU)
which is nice for a demonstration.
The following is the very straightforward program that I use.
Is it OK? I'm not asking for a faster sort algorithm,
but whether par/pseq is used correctly/idiomatically in this program
(so that students don't get a wrong impression).
msort  =  ; msort [x] = [x]
msort xs =
let ( here, there ) = splitAt ( div ( length xs ) 2 ) xs
mshere = msort here
msthere = msort there
in last mshere
`par` last msthere
`pseq` merge mshere msthere
merge  ys = ys ; merge xs  = xs
merge (x:xs) (y:ys) =
if x < y then x : merge xs (y:ys)
else y : merge (x:xs) ys
Thanks - J.W.
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