[Haskell-beginners] Random Numbers with the State Monad
Thomas Jakway
tjakway at nyu.edu
Fri Feb 12 01:14:08 UTC 2016
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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