I had a similar problem, but that was because I needed to use the constructors. MonadState has the methods you are using just write your code requiring MonadState as both State and StateT are instances. next_random_probability :: ( ..., MonadState g m) => m a

From: "Mark T.B. Carroll" To: haskell-cafe@haskell.org Subject: Writing for both State and StateT Date: Tue, 22 Apr 2003 07:48:57 -0400 (EDT)

At the moment I have code like,

next_random_probability :: (RandomGen g, Num a, Random a) => State g a

next_random_probability = do old_rng <- get let (random_probability, new_rng) = randomR (0, 1) old_rng put new_rng return random_probability

next_random_probabilityT :: (RandomGen g, Num a, Random a, Monad m) => StateT g m a

next_random_probabilityT = do old_rng <- get let (random_probability, new_rng) = randomR (0, 1) old_rng put new_rng return random_probability

Because these just use get and put which are both from MonadState, is it possible to write just one next_random_probability with a more general type signature, maybe mentioning MonadState, or at least two wrappers that call the same worker function, so that the same code is good for both State and StateT?

-- Mark

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