Re: [Haskell-beginners] Random Numbers with the State Monad

12 Feb 2016


      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 mailto:tjakway@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?
_______________________________________________
Beginners mailing list
Beginners@haskell.org mailto:Beginners@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners