ListT proposal for `transformers`

[Ben Millwood]

On Sat, Jul 13, 2013 at 08:47:02PM -0700, Gabriel Gonzalez wrote:
>I've been working on a "ListT done right" for submission into 
>`transformers`.  I could split it off into its own package, but I 
>would like to first run it by you all, particularly Ross, to see if 
>this can be included this in `transformers` because I feel that 
>`transformers` is where this belongs.

I'm not sure. I'm inclined to think that the reason that ListT's 
violation of the monad laws upsets no-one is because no-one actually 
uses it. I don't think we should include the transformer just for the 
sake of having one.

I've found myself using something like the new ListT before, but it's a 
bit awkward to have two new datatypes, and difficult to use existing 
list machinery with it, so I'm not sure it really pays off as an 
abstraction.

Do we have compelling use cases for either the old or the new ListT? Is 
the old ListT used anywhere on Hackage? In lieu of these things, I might 
propose just removing it altogether.