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

Dan Piponi dpiponi at gmail.com
Sun Oct 14 15:43:53 EDT 2007

On 10/14/07, Yitzchak Gale <gale at sefer.org> wrote:
> Not very much, I suspect. That "monad" really is broken -
> it's not a monad at all.

Depending on your point of view, ListT isn't broken. It correctly
transforms commutative monads into monads. The problem is that you
can't express "commutative monad" any differently from "monad" in
Haskell. And so it's been shoehorned into the wrong type class.

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

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