Generating random generators
 Lauri Alanko
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Date:   (:) 
From:  Lauri Alanko <la@i...> 
Subject:  Generating random generators 
To parallelize a probabilistic algorithm while retaining reproducibility, it is often useful to have several independent streams of pseudorandom numbers. In Haskell, there is a standard operation Random.split, which takes a PRNG state and produces two new PRNG states which should hopefully produce two seemingly unrelated number streams. OCaml's random library works in an imperative setting where each generation operation implicitly updates the state of the generator, so the corresponding operation would simply be generating a new random generator: val spawn : Random.State.t > Random.State.t Alas, such an operation is not provided by the library. The most straightforward implementation would be the following: let spawn s = Random.State.make (Array.init 55 (fun _ > Random.State.bits s)) However, random numbers are tricky, and I'm suspicious of just adding a new operation ad hoc when I don't understand how the underlying PRNG works. Hence, I'd appreciate if anyone could offer some insight on whether the above approach has any hidden pitfalls (i.e. some sort of regularity that might appear when the values from two generated streams are combined in a particular fashion), or if there is a faster way of generating new generators robustly. Random.State.make invokes Digest.string for every int of the seed, so it seems like overkill. Thanks in advance. Lauri