How to create a MonadRandom (StateT PureMT m0)? (Haskell)

Question

I am trying to use MonadRandom from https://hackage.haskell.org/package/random-fu-0.2.6.0/docs/Data-Random.html#g:6

Specifically, I have a function with the type signature:

randomN :: MonadRandom m => m Int

and I want to run it using the Mersenne Twister with a known seed.

How to I "create" an instance of Monad m0 => MonadRandom (StateT PureMT m0) from the documentation?

Answer 1

Since the instance for MonadRandom is already there for Monad m => StateT PureMT m, you just need something like

-- State s a = StateT s Identity a
test :: State PureMT (Int, Int)
test = do
    a <- randomN
    b <- randomN
    return (a, b)

And you can run it as

main :: IO ()
main = do
    -- You can replace 1234 with whatever seed you want
    let (result, finalState) = runState test $ pureMT 1234
    putStr "The result: "
    print result
    putStr "The final state: "
    print finalState