[Haskell-beginners] Random Numbers with the State Monad

[Nikita Kartashov]

Hi!

Take a look at MonadRandom [1]. It is basically what you want without getting generator explicitly.

[1] https://hackage.haskell.org/package/MonadRandom

With regards,
Nikita Kartashov



On 12 Feb 2016, at 04:14, Thomas Jakway <tjakway at nyu.edu> wrote:

> I'm having a bad time using the State monad to generate random numbers without carrying around a lot of StdGens manually.
> I have this snippet in the IO monad:
> 
>    ... IO stuff ...
>    gen <- getStdGen
>    let (numPlayers, numMatches) = (evalState genRandVariables gen) :: (Integer, Integer)
>    ... More IO stuff ...
> 
>    where maxRandPlayers = 10 :: Integer
>          minRandMatches = 10 :: Integer
>          maxRandMatches = 100 :: Integer
>          genRandVariables = (do
>                np <- randomR (1, maxRandPlayers) --minimum 1 other player
>                nm <- randomR (minRandMatches, maxRandMatches)
>                return (np, nm)) :: State StdGen (Integer, Integer)
> 
> 
> I get this error message:
> test/Jakway/Blackjack/Tests/IntegrationTests/MatchTests.hs:53:23:
>    Couldn't match expected type ‘StateT
>                                    StdGen Data.Functor.Identity.Identity Integer’
>                with actual type ‘g0 -> (Integer, g0)’
>    Probable cause: ‘randomR’ is applied to too few arguments
>    In a stmt of a 'do' block: np <- randomR (1, maxRandPlayers)
>    In the expression:
>        (do { np <- randomR (1, maxRandPlayers);
>              nm <- randomR (minRandMatches, maxRandMatches);
>              return (np, nm) }) ::
>          State StdGen (Integer, Integer)
> 
> test/Jakway/Blackjack/Tests/IntegrationTests/MatchTests.hs:54:23:
>    Couldn't match expected type ‘StateT
>                                    StdGen Data.Functor.Identity.Identity Integer’
>                with actual type ‘g1 -> (Integer, g1)’
>    Probable cause: ‘randomR’ is applied to too few arguments
>    In a stmt of a 'do' block:
>      nm <- randomR (minRandMatches, maxRandMatches)
>    In the expression:
>        (do { np <- randomR (1, maxRandPlayers);
>              nm <- randomR (minRandMatches, maxRandMatches);
>              return (np, nm) }) ::
>          State StdGen (Integer, Integer)
> 
> What's really baffling to me is I feel like this is how it *should* look--that the whole point of the state monad is to *not* have to explicitly pass the StdGen to randomR.  What am I doing wrong?
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