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.
If I were using the function in an executable I were writing, I would probably do something like unsafePerformIO . randomIO. Or thread in the random Gen from main if it were convenient. If I were writing it as a library function, I would leave the function as you described and let the caller make the choice. Calling into randomIO in a library function is extremely dubious, as a second library could be getting and setting the random seed used by randomIO (see setStdGen). So I'm okay taking on that risk in an application I write, but I'm not okay shipping that risk in a re-usable library, with the risk hidden behind a type signature. Maybe I'm just paranoid. Antoine