Add Applicative instances for MTL types

[Bas van Dijk]

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
>
> regards,
>
> Bas
>

getCurrentTime >>= \c -> print (realToFrac (diffUTCTime c (readTime
defaultTimeLocale "%c" "Sat Dec 20 19:09:00 2008")) / 60 / 60 / 24 /
7)

~ 3.5 weeks ago I sended this previous message ;-)

Can we have some discussion about my patch?

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

* Can we get rid of the Monad and MonadPlus constraints in the
Applicative and Alternative instances for StateT and RWST?

Thanks,

Bas