2009/1/1 Luke Palmer <lrpalmer@gmail.com>:

>
> So that's the answer: there already is a Strict monad. And an attempt to
> make a lazier one strict just results in breaking the monad laws.

There is at least one transformer that will make a strict monad out of
a non-strict monad.

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.

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?

Luke

> There's another answer though, regarding your question for why we don't just
> use StrictT State instead of a separate State.Strict. This message is
> already too long, and I suspect this will be the popular reply anyway, but
> the short answer is that Strict State is called that because it is strict in
> its state, not in its value.

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.

Luke