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Today's Topics:
1. Re: Random Numbers with the State Monad (Thomas Jakway)
2. Re: Random Numbers with the State Monad (Marcin Mrotek)
----------------------------------------------------------------------
Message: 1
Date: Fri, 12 Feb 2016 07:37:31 -0500
From: Thomas Jakway <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Random Numbers with the State Monad
Message-ID: <[email protected]>
Content-Type: text/plain; charset="windows-1252"; Format="flowed"
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 <[email protected]
> <mailto:[email protected]>> 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
>> [email protected] <mailto:[email protected]>
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
>
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Message: 2
Date: Fri, 12 Feb 2016 14:26:36 +0100
From: Marcin Mrotek <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Random Numbers with the State Monad
Message-ID:
<cajcfpzntxtzi6+mlr_tzgfsh8oqanj6pmduyofnvd0yq70g...@mail.gmail.com>
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Hello,
You have this type error:
Couldn't match expected type ?StateT
StdGen
Data.Functor.Identity.Identity Integer?
with actual type ?g1 -> (Integer, g1)?
It's because you're trying to use a `(s -> (a,s))` function
"unwrapped" where GHC expects it "wrapped" in a StateT.
The type of `state` from Control.Monad.State (mtl package) is:
state :: (s -> (a, s)) -> m a
so it could solve your mismatch, turning `g1 -> (Integer, g1)` into
`StateT StdGen Identity Integer`, like:
np <- state (randomR (1, maxRandPlayers))
Alternatively, if you don't want to use mtl, you can use `StateT`s
constructor directly. It's type is:
StateT :: (s -> m (a, s)) -> StateT s m a
so you'd have to compose `randomR` with `return` (or `pure`) first:
np <- StateT (return . randomR (1, maxRandPlayers))
Best regards,
Marcin Mrotek
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