2008/10/13 Daryoush Mehrtash
Is there a write up on what makes an implementation lazy vs strict?
I would be interested in seeing this, too!
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?
Here's one I can answer. ~ causes Haskell to do the pattern match lazily. Normally, when you write (a,_) <- runStateT m s it eagerly evaluates the runStateT m s far enough to determine that the result is in fact a tuple in the underlying monad (as opposed to, say, fail). Adding the ~ tells Haskell to not do any computation until it actually needs the value of a. You can find a better explanation and examples at http://en.wikibooks.org/wiki/Haskell/Laziness Cheers, steve