Add Applicative instances for MTL types

[Duncan Coutts]

On Wed, 2009-01-14 at 13:54 +0100, Bas van Dijk wrote:
> On Sat, Dec 20, 2008 at 7:09 PM, Bas van Dijk <v.dijk.bas at gmail.com> wrote:
> > Hello,
> >
> > In a project of mine I needed an Applicative instance for Identity and
> > noticed it didn't exist. So I decided to add Applicative (and
> > Alternative instances where possible) for all MTL types.
> >
> > When I was about to submit a library proposal I saw there already
> > existed one. So I added my patch to that ticket.
> >
> > My patch I different in that my Applicative instances don't require a
> > Monad constraint. This also means that most Functor instances now also
> > depend on Applicative rather than on Monad.
> >
> > See the ticket for the details:
> >
> > http://hackage.haskell.org/trac/ghc/ticket/2316
> >
> > Discussion period: 5 weeks (taking the holidays into account)
> >
> > This is the old thread about this proposal:
> >
> > http://thread.gmane.org/gmane.comp.lang.haskell.libraries/9140/focus=9154


> ~ 3.5 weeks ago I sended this previous message ;-)
> 
> Can we have some discussion about my patch?

Yes please!

I think we all want all the MTL monads to be instances of Applicative.
I'm not sure if I am qualified to comment on the details of the patch
however.

If we don't get enough reviewers, we should try pinging people, like the
original MTL authors, authors of similar mtl-esque packages etc.

> As listed in the ticket I have these questions:
> 
> * Does it make sense adding Applicative counterparts to the Monad*
> classes? For example:
>   o class Applicative f => ApplicativeError e g | f -> e where ...
>   o class Applicative f => ApplicativeState s f | f -> s where ...
>   o class Applicative f => ApplicativeReader r f | f -> r where ...
>   o class (Applicative f, Monoid w) => ApplicativeWriter w f | f -> w where ...
>   o class (Monoid w, ApplicativeReader r f, ApplicativeWriter w f,
> ApplicatveState s f) => ApplicativeRWS r w s f | f -> r, f -> w, f ->
> s where ...
>   o class (Applicative m) => ApplicativeCont m where ...
>   o class ApplicativeTrans t where lift :: Applicative f => f a -> t f a

That sounds like it wants to be an independent proposal.

Duncan