[Haskell-cafe] ListT considered harmful

Ross Paterson R.Paterson at city.ac.uk
Fri Mar 27 10:23:03 UTC 2015

On Fri, Mar 27, 2015 at 01:03:04PM +0900, Fumiaki Kinoshita wrote:
> It is well known that ListT m in transformers is not a monad unless
> the underlying monad is commutative. The fact that ListT is not a
> monad transformer is sufficient to remove it from the package, and
> several packages already defined their own ListT.
> Oughtn't we deprecate ListT or replace it by a correct one?

This flaw is at least documented, and it does severely limit the
usefulness of ListT.  Deprecation seems reasonable, but we'd need to
offer a replacement first.

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