[Haskell-beginners] Random Numbers with the State Monad

[Marcin Mrotek]

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