On Tue, Jan 30, 2007 at 04:58:48PM +0000, Ian Lynagh wrote:
I propose we make StateT lazy
SimonM writes:
I don't mind whether StateT itself is lazy or not, but I think we should have a strict version. In GHC the implementation should use unboxed tuples, like IO.
If we're going to have a strict StateT then it would make sense to have a strict State too. Control.Monad.State.Strict perhaps? (and likewise for the other mtl monads, where appropriate). Or perhaps go a bit further and have: Control.Monad.State.Class -- for the MonadState class Control.Monad.State.Lazy Control.Monad.State.Strict Control.Monad.State -- re-exports .Class and .Lazy There's perhaps an argument for splitting the package into mtl-classes, mtl-lazy and mtl-strict but I think that that would be overkill at this stage. There's going to be a lot of duplication between the lazy and strict (and GHC) variants, but short of some hideous CPP I don't think there's a nice solution. By "strict", do you mean this?: 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