[Haskell-cafe] Applicative and Monad transformers

[Jeremy Shaw]

At Thu, 27 Aug 2009 10:47:43 -0400,
Job Vranish wrote:

> I've  often wanted an applicative instance for a datatype that didn't match
> the monad instance.

> It would be nice if there was a way to hide instances so that
> they could be redefined.

Yeah, this is similar to the issue of multiple sensible instances for
Data.Monoid. This issue has come up several times for me recently. A
bit of a shortcoming in the type class design I think.

There was once a proposal to help address this, but it never gained
tracation:

http://scholar.google.com/scholar?hl=en&lr=&cluster=8306039955712318110&um=1&ie=UTF-8&ei=JTmXSvmTJ4a4M6TJlfkN&sa=X&oi=science_links&resnum=1&ct=sl-allversions

Recently I have wondered if module functors might somehow be a
solution somehow. But I have not actually investigated at all.

 - jeremy