Hi, Roman —<div><br></div><div>Recently, I switched the mwc-random package (<a href="http://hackage.haskell.org/package/mwc-random">http://hackage.haskell.org/package/mwc-random</a>) over from running in the ST monad to using your primitive package. I didn&#39;t notice initially, but this caused a huge performance regression.</div>
<div><br></div><div>mwc-random 0.4.1.1 uses ST internally, and runs the benchmarks/Quickie benchmark in 0.017 seconds.</div><div><br></div><div>mwc-random 0.5.1.3 uses PrimMonad internally, and takes 1.328 seconds to run the same benchmark.</div>
<div><br></div><div>That&#39;s about a factor of 80 slowdown. This causes a massive knock-on performance loss in packages such as criterion that need fast PRNGs.</div><div><br></div><div>The problem is very easy to reproduce:</div>
<div><br></div><div>cabal install mwc-random-0.4.1.1</div><div>cabal install mwc-random-0.5.1.3</div><div>darcs get <a href="http://darcs.serpentine.com/mwc-random/">http://darcs.serpentine.com/mwc-random/</a></div><div>cd mwc-random/benchmarks</div>
<div>ghc -fforce-recomp -O  -package mwc-random-0.4.1.1 --make Quickie -o quickie-411</div><div>ghc -fforce-recomp -O  -package mwc-random-0.5.1.3 --make Quickie -o quickie-513</div><div><br></div><div>time ./quickie-411</div>
<div>time ./quickie-513</div><div><br></div><div>If you could shed any light, I&#39;d be most grateful, as this has me a bit confounded.</div>