mwc-random / README.markdown

Efficient, general purpose pseudo-random number generation

This package provides the System.Random.MWC module, a Haskell library for generating high-quality pseudo-random numbers in a space- and time-efficient way.


This library has been carefully optimised for high performance. To obtain the best runtime efficiency, it is imperative to compile libraries and applications that use this library using a high level of optimisation.

Suggested GHC options:

-O -fvia-C -funbox-strict-fields

To illustrate, here are the times (in seconds) to generate and sum 250 million random Word32 values, on a laptop with a 2.4GHz Core2 Duo P8600 processor, running Fedora 11 and GHC 6.10.3:

no flags   200+
-O           1.249
-O -fvia-C   0.991

As the numbers above suggest, compiling without optimisation will yield unacceptable performance.

Get involved!

Please report bugs via the bitbucket issue tracker.

Master Mercurial repository:

  • hg clone

There's also a git mirror:

  • git clone git://

(You can create and contribute changes using either Mercurial or git.)


This library is written and maintained by Bryan O'Sullivan,