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

Yitzchak Gale gale at sefer.org
Sun Oct 14 16:12:28 EDT 2007

Hi Dan,

I wrote:
>> That "monad" really is broken - it's not a monad at all.

Dan Piponi wrote:
> Depending on your point of view, ListT isn't broken. It correctly
> transforms commutative monads into monads.

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?

> The replacement ListT is usually the monad transformer
> I need when I want a List-like monad transformer,
> so I'm all for it being in (the standard prelude) mtl.

OK. I take this as a +1 vote for the rewrite.

> But it seems sad there isn't a place for the old one
> to live.

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


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