[Haskell-beginners] bad state monad instances

[Daniel Fischer]

On Wednesday 23 June 2010 03:53:14, Keith Sheppard wrote:
> Hi,
>
> I'm working on understanding the state monad, and I got stumped pretty
> much right away. When I run the following script (with instances
> copied verbatim from
> http://www.haskell.org/all_about_monads/html/statemonad.html )
>
> #!/usr/bin/env runhaskell
> \begin{code}
> {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
> import Control.Monad.State(Monad, MonadState(..))
>
> newtype State s a = State { runState :: (s -> (a,s)) }
>
> instance Monad (State s) where
>     return a        = State $ \s -> (a,s)
>     (State x) >>= f = State $ \s -> let (v,s') = x s in runState (f v)
> s'
>
> instance MonadState (State s) s where
>     get   = State $ \s -> (s,s)
>     put s = State $ \_ -> ((),s)
>
> main :: IO ()
> main = putStrLn "hello"
>
> \end{code}
>
>
> It fails with:
> statemonadtest.lhs:11:20:
>     `State s' is not applied to enough type arguments
>     Expected kind `*', but `State s' has kind `* -> *'
>     In the instance declaration for `MonadState (State s) s'
>
> Can you see what I'm doing wrong? I must be making a really basic
> mistake but I'm not sure what it is.

Wrong argument order in the MonadState instance,

class (Monad m) => MonadState s m | m -> s where
  get :: m s
  put :: s -> m ()

The state type comes first, then the Monad.

Make it

instance MonadState s (State s) where ...

I don't know if that's been changed at some point or if it was a typo in 
the tutorial from the beginning.

>
> Thanks, Keith