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 wrote:

On Mon, 2008-10-13 at 13:37 -0700, Daryoush Mehrtash wrote:

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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).

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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

Hello, On Mon, Oct 13, 2008 at 3:16 PM, Stephen Hicks wrote:

2008/10/13 Daryoush Mehrtash :

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Is there a write up on what makes an implementation lazy vs strict?

I would be interested in seeing this, too!

Typically it has to do with the strictness of the "bind" operation. Here is an example to illustrate the difference. evalState (undefined >> return True) undefined When you evaluate this in the context of Control.Monad.State, the result is "True", while if you evaluate it in the context of Control.Monad.State.Strict you will get "undefined". It may be interesting to compare MTL's approach to what's done in monadLib (another monad transformer library). In monadLib, transformers like state inherit the strictness of their bind operation from the underlying monad. There are two base level monads: Id, which is lazy, and Lift which is strict. So to get a strict state monad in monadLIb, you would write "StateT s Lift", and to get the lazy version you would use "StateT s Id". Hope that this helps, Iavor