Re: Proposal: Make StateT in mtl lazy

1 Feb 2007


      On Wed, Jan 31, 2007 at 03:48:25PM +0000, Ian Lynagh wrote:
...
instance Monad (State s) where
    return a = State $ \s -> (a, s)
    m >>= k  = State $ \s -> case runState m s of
                             (a, s') -> runState (k a) $! s'
instance (Monad m) => Monad (StateT s m) where
    return a = StateT $ \s -> return (a, s)
    m >>= k  = StateT $ \s -> do
        (a, s') <- runStateT m s
        runStateT (k a) $! s'
    fail str = StateT $ \_ -> fail str
We agreed on IRC that the $!'s solve an orthogonal problem so the strict
monads will just have:

instance Monad (State s) where
    return a = State $ \s -> (a, s)
    m >>= k  = State $ \s -> case runState m s of
                             (a, s') -> runState (k a) s'

instance (Monad m) => Monad (StateT s m) where
    return a = StateT $ \s -> return (a, s)
    m >>= k  = StateT $ \s -> do
        (a, s') <- runStateT m s
        runStateT (k a) s'
    fail str = StateT $ \_ -> fail str


Thanks
Ian