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.