[Haskell-cafe] Re: Help mixing pure and IO code

Luke Palmer wrote:

Hector Guilarte wrote:

...

f:: [int] -> (a,[Int]) f randomList = let (usedRandomNumber,newRandomList) = g randomList in (usedRandomNumber,newRandomList)

This pattern can be encapsulated in a monad:

newtype RandM a = RandM { unRandM :: [Int] -> (a,[Int]) }

instance Monad RandM where [...]

See the similarity?

Of course, there is no need to implement this yourself. It is already implemented as State [Int]. And as long as you are doing that, you might as well use Rand from the MonadRandom package. In fact, I have argued that you should use MonadRandom instead of the lower-level System.Random whenever possible: http://lukepalmer.wordpress.com/2009/01/17/use-monadrandom/

The rationale being that RandM a has a natural interpretation as "random variable of type a" with no reference to how it's actually implemented. Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com