2009/7/6 Matthias Görgens <matthias.goergens@googlemail.com>
A Las Vegas algorithm, like randomized quicksort, uses a source of
randomness to make certain decisions. However its output is
unaffected by the randomness. So a function
> f :: RandomGen g => g -> a -> b
implementing a Las-Vegas-Algorithm 'looks' like a pure function,
ignoring its first argument and depending solely on its second
argument. What is an idiomatic way to implement such a function? I
believe, Monads are too linear and strict.
Interesting question!
Well, you could make your own random generator for the lifetime of the function, with a fixed seed. I'd say this is the most "honest" way to do it; however, might a malicious user discover your seed, he could design an input that would make your algorithm perform poorly.
I'm wary of saying you could use unsafePerformIO . randomRIO to get a seed. But I think some sort of unsafe something has to be involved, since you are representing a very advanced proof obligation (the algorithm is independent of the randomness).
Keep us (me) posted on developments on this idea.
Luke