On 12 Feb 2016, at 04:14, Thomas Jakway <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?
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