[Haskell-cafe] [random-fu] sampleRVar with StdGen in RWS?

[Vanessa McHale]

It seems like you need your write-state to be a monoid in order to infer
a `MonadRandom` instance. Based on your code, I'm guessing the compiler
can't infer that.

Have a look at
https://hackage.haskell.org/package/MonadRandom-0.5.1.1/docs/Control-Monad-Random-Class.html#t:MonadRandom


On 08/03/2018 07:31 AM, Sergiu Ivanov wrote:
> Dear Café,
>
> I would like to use random-fu to do some pseudo-random simulations for a
> given StdGen (so that I can run the same simulation multiple times, if
> needed).
>
> The following works:
>
>   testState :: StdGen -> Int
>   testState = evalState (sampleRVar $ uniform 1 10)
>
> The following doesn't:
>
>   testRWS :: StdGen -> Int
>   testRWS = fst . evalRWS (sampleRVar $ uniform 1 10) ()
>   
> I get
>
>   <interactive>:2:26: error:
>   • No instance for (MonadRandom (RWST () b0 StdGen Identity)) arising from a use of ‘sampleRVar’
>   • In the first argument of ‘evalRWS’, namely ‘(sampleRVar $ uniform 1 10)’
>   In the second argument of ‘(.)’, namely ‘evalRWS (sampleRVar $ uniform 1 10) ()’
>   In the expression: fst . evalRWS (sampleRVar $ uniform 1 10) ()
>
> Indeed, I do see a MonadRandom instance for StateT, but none for RWST [0].
>
> Is there a reason to not have a MonadRandom instance for RWST?
>
> Am I looking in the wrong place?
>
> -
> Sergiu
>
> [0] https://hackage.haskell.org/package/random-source-0.3.0.6/docs/Data-Random-Source.html
>
>
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-- 



*Vanessa McHale*
Functional Compiler Engineer | Chicago, IL

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