This module contains a permuted linear congruential pseudorandom number generator, as described by M.E. O'Neill (pcg-random.org). This version holds two Word64 values and outputs a Word32 of randomness each time you use it. Compared to the StdGen type from System.Random, it's around a 2.5x to 3x speedup on a 64-bit system. It runs somewhat slower than StdGen on a 32-bit system (the Word64 values must be emulated after all), but there aren't too many of those in play these days anyway. Raspberry Pi is the only common thing I can think of, which isn't the best for Haskell in the first place. If you somehow are using 32-bit all the time I guess this module isn't for you.

The first Word64 is the state, which changes with each step of the generator. The second Word64 is the inc, which stays fixed from step to step and controls what stream of numbers is generated. The inc is always bitwise OR'd with 1 during computations, so there are only 2^63 distinct number streams (eg: inc 2 and 3 are the same number stream). The state value eventually returns to it's initial value after 2^64 uses and the whole sequence loops around.