Hi, Roman — Recently, I switched the mwc-random package ( http://hackage.haskell.org/package/mwc-random) over from running in the ST monad to using your primitive package. I didn't notice initially, but this caused a huge performance regression. mwc-random 0.4.1.1 uses ST internally, and runs the benchmarks/Quickie benchmark in 0.017 seconds. mwc-random 0.5.1.3 uses PrimMonad internally, and takes 1.328 seconds to run the same benchmark. That's about a factor of 80 slowdown. This causes a massive knock-on performance loss in packages such as criterion that need fast PRNGs. The problem is very easy to reproduce: cabal install mwc-random-0.4.1.1 cabal install mwc-random-0.5.1.3 darcs get http://darcs.serpentine.com/mwc-random/ cd mwc-random/benchmarks ghc -fforce-recomp -O -package mwc-random-0.4.1.1 --make Quickie -o quickie-411 ghc -fforce-recomp -O -package mwc-random-0.5.1.3 --make Quickie -o quickie-513 time ./quickie-411 time ./quickie-513 If you could shed any light, I'd be most grateful, as this has me a bit confounded.