Let's do ListT right, finally

Yitzchak Gale gale at sefer.org
Sun Oct 14 15:00:59 EDT 2007

Hi Ian, thanks for responding to my plea!

I am renaming this thread and moving it to libraries.
Please respond there.

I wrote:
>> http://haskell.org/haskellwiki/ListT_done_right
>> and not the broken implementation that comes with mtl.
>> (Finally fixed in 6.8? Please?)

Ian Lynagh wrote:
> If you want to propose that mtl switches to a different definition of
> ListT then please see
>     http://www.haskell.org/haskellwiki/Library_submissions

OK, then. This is a major rewrite of this module.
So let me open up the topic for discussion even
before posting a submission.

Do we need to worry about breaking code?

Not very much, I suspect. That "monad" really is broken -
it's not a monad at all. The problem and its solution have
been well known for years. I personally tried using the
"standard" ListT for a while, just because it's the standard.
Until I ran into enough problems that I finally slapped my
forehead and said "forget it".

But just in case: Is there anyone reading this who would
be hurt by this change and opposes it? Should we provide
a more gradual deprecation path (what is it)?


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