[Haskell-beginners] random monad

[Dennis Raddle]

I'm writing a program which uses a lot of pseudorandom numbers, and for
that reason it would be nice to put the StdGen in a state monad.

like let's say I want to combine error handling and storing the StdGen
state.

import Control.Monad.Error
import Control.Monad.State

data RandState = RandState StdGen
-- Er is a monad that combines error handling and pseudorandom state
type Er a = ErrorT String (State RandState) a

-- to access random numbers, I could define things like


erRandomR :: Random r => (r,r) -> Er r
erRandomR (lo, hi) = do
  RandState g <- get
  let (value, g') = randomR (lo, hi) g
  put $ RandState g'
  return value


erRandoms :: Random r => Er [r]
erRandoms = do
  RandState g <- get
  let (g1, g2) = split g
  let values = randoms g1
  put $ RandState g2
  return values


erRandomRs :: Random r => (r,r) -> Er [r]
erRandomRs (lo,hi) = do
  RandState g <- get
  let (g1, g2) = split g
  let values = randomRs (lo,hi) g1
  put $ RandState g2
  return values

-- I could define new ways of using random values, like choosing a random
element of a list

erChooseList :: [a] -> Er a
erChooseList xs = do
  let l = length xs
  when (l==0) (throwError "in randomChooseList, passed null list")
  idx <- erRandomR (0,l-1)
  return $ xs !! idx

However, after I got done with that, I realized that I wanted to add
additional state, maybe a ReaderT , stuff like that--different in different
parts of the program. But I always want access to random numbers with the
same functions: erRandomR, erRandoms, etc.

So I thought

class Monad m => RandMonad m where
  putGen ::  StdGen -> m ()
  getGen :: m StdGen

Then I could make Er an instance of RandMonad, like this

type Er a = ErrorT String (State RandState) a
instance RandMonad Er where
  putGen g = put (RandState g)
  getGen = do RandState g <- get
              return g

Clearly I don't know what I'm doing, because when I tried to run this much
I got the error

"Type synonym Er should have 1 argument, but has been given none."

I tried a bunch of variations of this but got nowhere. Can someone explain
how I should conceive of this?
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