[Haskell-cafe] Implementing Las Vegas algorithms in Haskell
aslatter at gmail.com
Tue Jul 7 02:10:01 EDT 2009
2009/7/6 Matthias Görgens <matthias.goergens at 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.
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
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.
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