[Haskell-cafe] Convert IO Int to Int

[Daniel Fischer]

Am Dienstag 09 Juni 2009 15:57:24 schrieb Magnus Therning:
> On Tue, Jun 9, 2009 at 2:52 PM, ptrash<ptrash at web.de> wrote:
> > Hmm...it am not getting through it. I just want to generate a random
> > number and then compare it with other numbers. Something like
> >
> > r = randomRIO (1, 10)
> > if (r > 5) then... else ...
>
> You have to do it inside the IO monad, something like
>
>     myFunc  = do
>         r <- randomRIO (1, 10
>         if r > 5
>             then ...
>             else ...
>
> /M

Or make the source of the pseudo-random numbers explicit:

import System.Random

function :: (RandomGen g, Random a) => g -> other args -> result
function gen whatever
    | r > 5     = blah newgen something
    | r < 3     = blub newgen somethingElse
    | otherwise = bling
      where
        (r,newgen) = randomR (lo,hi) gen

and finally, when the programme is run:

main = do
    args <- getArgs
    sg <- getStdGen
    foo <- thisNThat
    print $ function sg foo

If you're doing much with random generators, wrap it in a State monad.