[Haskell-beginners] bad state monad instances

Keith Sheppard keithshep at gmail.com
Tue Jun 22 21:53:14 EDT 2010


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
{-# 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"


It fails with:
    `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

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