[Haskell-cafe] Let's do ListT right, finally

[Dan Piponi]

On 10/14/07, Yitzchak Gale <gale at sefer.org> wrote:
> Interesting. What do you mean by a "commutative monad"?
> It can't be a monad with some sort of additional commutative
> law, because the old ListT doesn't even satisfy the monad
> laws. Or does it in some sense?

If m is a commutative monad, then ListT m is a fully paid up monad,
not just "in some sense".

On a brief definition of commutative monads, and for some examples
indicating how common they are, see SPJ's
http://research.microsoft.com/~simonpj/papers/haskell-retrospective/HaskellRetrospective-2.pdf

> OK, do you propose that we keep it and change its name?

+1, dump the old one completely because you can't express what kind of
thing the old ListT is in Haskell. I just want to make sure it gets a
good epitaph. If someone answers SPJ's challenge #2 maybe it'll come
back.
--
Dan