2009/7/6 Matthias Görgens
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