Is there a write up on what makes an implementation lazy vs strict?
I like to better understand the trade off between the two and use cases
where one is better than the other.
I noticed that some functions in the lazy implementation uses "~" . For
example
evalStateT :: (Monad m) => StateT s m a -> s -> m a
evalStateT m s = do
~(a, _) <- runStateT m s
return a
What does ~ do?
thanks,
Daryoush
On Mon, Oct 13, 2008 at 1:34 PM, Jonathan Cast
On Mon, 2008-10-13 at 13:37 -0700, Daryoush Mehrtash wrote:
Question 1: Why are there lazy and strict modules of some monads? (e.g. Control.Monad.State)
Because both are useful, for different purposes. (For the same reason that it's helpful, in general, to have both eager and lazy evaluation in the same language --- sometimes one version is more efficient, sometimes the other one is).
Question 2: If I define a new monad (say XYZ), does it have to be as Control.Monad.XYZ module?
No. Haskell has neither a requirement nor a convention that monads go in Control.Monad. Control.Monad.* is simply the home of the MTL library, which contains a number of exceptionally useful monads; new monads that aren't as exceptionally general-purpose as MTL probably shouldn't go there, to reduce clutter.
jcc