ListT proposal for `transformers`

[Edward Z. Yang]

ListT is a kind of delicate monad, because it has interesting
control flow (arguably one is supposed to use LogicT for anything
serious, since ).  What makes this particular implementation "ListT
done right?"

Edward

Excerpts from Gabriel Gonzalez's message of Sat Jul 13 20:47:02 -0700 2013:
> 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've lpasted the implementation here:
> 
> http://lpaste.net/90890
> 
> I've written it to follow the spirit of the `transformers` library as 
> closely as possible, both in API, documentation, and source code style.
> 
> I'm fine with either replacing the old `ListT` implementation or 
> providing both.  I'll leave it to you guys to make the call on that.  As 
> long as the correct version is in `transformers` somewhere then I'm 
> happy.  If I *had* to pick a side, I suppose I would favor replacing the 
> old one.
> 
> Also, any suggestions for changes are welcome, particularly if they 
> improve the likelihood of this getting into `transformers`.
>