Comments on: Are continuations really the mother of all monads?

By: Mauro Jaskelioff

Mauro Jaskelioff — Thu, 24 Nov 2011 15:03:59 +0000

I’d like to clarify the comment by lpsmith. My point in Modular Monad Transformers, is that the lifting in the MTL of callCC through StateT behaved differently than other liftings of callCC in the library (that’s why I argue is not the correct one). Furthermore I provided a uniform way of lifting callCC through *any* monad transformer, which I think is the correct, uniform, way of doing it.
Consequently, lifting callCC (as defined in the paper) should be a simple matter of post-composing lift.

By: lpsmith

lpsmith — Sun, 31 Jul 2011 16:09:30 +0000

In a conversation with Edward Kmett, he suggested that the Lazy State Monad is, arguably, not a monad in the categorical sense, because fmap id ≠ id. In particular, fmap id ⊥ = λ _ → (⊥,⊥).

By: lpsmith

lpsmith — Sat, 06 Feb 2010 21:47:20 +0000

It was pointed out to me by Russell O'Connor, Dan Doel, and others that StateT st (Cont r) is in fact the exact same monad as ContT r (State st) in the context of the Monad Transformer Library. However, Mauro Jaskelioff in his paper Modular Monad Transformers argues in Section 4 that it is more natural for callCC to be lifted differently in the StateT st (Cont r) case, so that it differs from the instance of callCC for ContT r (State st). According to Jaskelioff, the MTL's lifting is correct in the second case, but not in the first. The two liftings are also mentioned in Monad Transformers and Modular Interpreters in Section 8.3, which includes a few other references to where each has appeared in the literature.

By: Noam Zeilberger

Noam Zeilberger — Tue, 22 Dec 2009 20:22:39 +0000

> One thing is clear, however: its tricky to generalize Filinski’s Representing Monads to lazy evaluation, if indeed it is possible to do so fully. You should read Lazy Evaluation and Delimited Control by Garcia, Lumsdaine and Sabry. I don't fully understand the paper yet, but I think it gives some sort of affirmative answer.