Change the instance head to
instance MonadState s (State s) where
It looks like the tutorial has the two parameters for MonadState in
the opposite order as does the mtl package.
Alex
On Tue, Jun 22, 2010 at 6:53 PM, Keith Sheppard
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.
Thanks, Keith -- keithsheppard.name _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners