ListT proposal for `transformers`

Ross Paterson R.Paterson at
Sun Jul 21 02:18:18 CEST 2013

On Sun, Jul 14, 2013 at 07:37:21AM -0700, Gabriel Gonzalez wrote:
> On 07/14/2013 03:44 AM, Ben Millwood wrote:
> >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.
> >
> This is a chicken-and-egg problem.  Nobody uses it because it
> violates the laws and there is no good `ListT` implementation on
> Hackage.

It seems there are 140 packages on hackage using ListT (which surprised
me, given its flaws), so I don't think we can quickly remove it from
transformers or re-implement it under the same name.

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