[Haskell-cafe] Lazy MWC Random?

Sacha Sokoloski sacha404 at gmail.com
Tue Jan 28 18:08:14 UTC 2014

Dear Haskellers,

I'm in a situation where I'd like to generate an infinite list of random 
elements (basically, I'm simulating stochastic systems). I feel like MWC 
Random is the fastest RNG available, but when I try to pull the infinite 
list out of RandST, it obviously doesn't halt, because ST is strict. 
Someone posted a way around this in this stack overflow thread:


Which would fix my problem. My question is though, why isn't ST.Lazy 
included as a PrimMonad instance anyway? The best answer I can come up 
with is that, since evaluating the Generator is time dependent, it's 
best to make it strict to make sure that one's program isn't tapping 
into /dev/random at arbitrary times.

In this way the best stackoverflow solution is quite good. It requires 
one to strictly generate a Seed (since that's the only way to do it), 
but then converts the ST Monad to the Lazy version to Lazify everything 
else. However, my understanding of PrimMonad is simply that it's a class 
of low level monads i.e. IO and ST, so if there's some deeper reason to 
this, it's beyond me.

Another question that I'm puzzling over: In the stack overflow solution, 
they also make an effort to only have to generate the seed a single 
time. Is this important performance wise? What I suppose this must hinge 
upon, is whether in saving an ST s Gen to a Seed, the conversion from an 
immutable to mutable array requires a copy or not. Is that the full 
extent of the complexity of this? Is the stackoverflow solution 
ultimately the most efficient? Is using MWC Random to generate infinite 
lists and efficient solution anyway?

Thanks for any insight,

  - Sacha Sokoloski

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