[Haskell-beginners] Random Numbers with the State Monad

[Thomas Jakway]

That's a good idea, though not understanding the state monad is still 
pretty frustrating.

On 2/12/16 6:38 AM, Nikita Kartashov wrote:
> 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 
> <mailto: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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>
>
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