Re: [Haskell-cafe] Stupid question #852: Strict monad

On Thu, Jan 1, 2009 at 3:44 PM, Luke Palmer wrote:

On Thu, Jan 1, 2009 at 1:31 PM, David Menendez wrote:

...

newtype CPS m a = CPS { unCPS :: forall b. (a -> m b) -> m b }

I have heard this called the "codensity monad" (and it appears under that name in category-extras). Good observation.

Interesting. I hadn't heard that name for it before. In my own monad library, I called it CPS, since it transforms a monad into continuation-passing style. It's handy for monads with an expensive (>>=), as discussed in Janis Voigtlander's "Asymptotic Improvement of Computations over Free Monads".

In my reply I missed the important consideration of the strictness of (>>=), irrsepective of the values. While you can not force values to be strict in a monad without breaking a law, (>>=) is "up for grabs", and that's what people are referring to when they refer to strict and lazy monads.

So I guess "strict monad" means (>>= f) is strict for all f. Right?

That's my understanding. As a consequence, strict monads in Haskell also have the property that "return undefined" never equals "undefined". Otherwise, they wouldn't satisfy the monad laws.

...

No, Control.Monad.State.Strict and Control.Monad.State.Lazy never force evaluation of their states.

Control.Monad.State.Strict> evalState (put undefined) '0' ()

My mistake.

I made that mistake myself, once. It doesn't help that the documentation just says "Lazy state monads" and "Strict state monads" without any further explanation. -- Dave Menendez http://www.eyrie.org/~zednenem/