Add Applicative instance for STM

[Bas van Dijk]

The deadline for the proposal to add Applicative and Alternative
instances for STM has passed. Nobody rejected and some people
accepted.

It would be great if somebody can apply the patches:

* http://hackage.haskell.org/trac/ghc/attachment/ticket/4076/stm_applicative-instance-sequential-stm.dpatch

* http://hackage.haskell.org/trac/ghc/attachment/ticket/4076/base_add_applicative_and_alternative_instances_for_STM_to_Control.Applicative.dpatch

Thanks,

Bas

On Tue, May 25, 2010 at 1:20 AM, Bas van Dijk <v.dijk.bas at gmail.com> wrote:
> Hello,
>
> This is just a reminder and summary of my proposal to add Applicative
> and Alternative instances for STM. We're in the second and last week
> of this proposal.
>
> Ticket: http://hackage.haskell.org/trac/ghc/ticket/4076
>
> Here's a description of the proposed patches:
>
> * stm_applicative-instance-sequential-stm.dpatch:
> This patch adds an Applicative instance for Control.Sequential.STM.
> Note that it doesn't add an Alternative instance because sequential
> STM does not support retrying transactions.
>
> Now for the regular STM there are actually two modules where we can
> define the Applicative and Alternative instances: GHC.Conc (which
> defines the type) and Control.Applicative (which defines the classes).
>
> The former has the advantage that the Functor, Applicative and Monad
> instances are grouped together. The disadvantage is that it requires
> to import Control.Applicative via {-# SOURCE #-} because we get an
> import cycle otherwise and so also requires the
> Control/Applicative.hs-boot file. The following patch makes this
> change:
> * base_applicative-instance-stm.dpatch
>
> More people like to add the instances to Control.Applicative however.
> The following patch makes that change:
> * base_add_applicative_and_alternative_instances_for_STM_to_Control.Applicative.dpatch
>
> Note there's a related proposal with patches for moving the MonadPlus
> instance for STM from Control.Monad.STM to GHC.Conc to avoid an
> orphaned instance. See:
> http://hackage.haskell.org/trac/ghc/ticket/4077
>
> One last thing, I actually used to different Applicative instances for STM:
>
> The one in Control.Applicative:
> instance Applicative STM where
>    pure = return
>    (<*>) = ap
>
> Or the one in GHC.Conc:
> instance Applicative STM where
>    {-# INLINE pure  #-}
>    {-# INLINE (<*>) #-}
>    pure  = returnSTM
>    (<*>) = appSTM
>
> returnSTM :: a -> STM a
> returnSTM x = STM (\s -> (# s, x #))
>
> appSTM :: STM (a -> b) -> STM a -> STM b
> appSTM (STM mf) (STM mx) = STM ( \s ->
>  case mf s of
>    (# s', f #) -> case mx s' of
>                     (# s'', x #) -> (# s'', f x #)
>  )
>
> Currently I don't have time to investigate if one is more efficient
> than the other. Does anybody have a hunch?
>
> Regards,
>
> Bas
>