Proposal: merge either into transformers

[Ross Paterson]

On Thu, Apr 24, 2014 at 06:14:48PM +0300, Roman Cheplyaka wrote:
> * Vincent Hanquez <tab at snarc.org> [2014-04-24 15:45:34+0100]
> > On 2012-12-07 09:44, Roman Cheplyaka wrote:
> > >I propose to add the sole module of the 'either' package[1],
> > >Control.Monad.Trans.Either, to the transformers package.
> > >
> > >It provides EitherT, a very basic and fundamental data type. The
> > >difference between EitherT and ErrorT is that the latter has an Error
> > >constraint, which is used to imlement 'fail'.
> > >
> > >Note that 'either' depends on the 'semigroupoids' and 'semigroup'
> > >packages to provide appropriate instances. The proposal is not to add
> > >those instances to 'transformers' to avoid additional dependencies. The
> > >instances can then be left in the 'either' package or moved to the
> > >'semigroupoids' and 'semigroup' packages respectively. ('semigroupoids'
> > >already depends on 'transformers', while 'semigroups' does not).
> > >
> > >Compared to the 'either' package, Show, Read, Eq and Ord instances will
> > >be dropped to keep the code Haskell2010 (those instances require
> > >FlexibleInstances, FlexibleContexts, and UndecidableInstances).
> > >
> > >The patch is attached. [*]
> > >
> > >[*] against transformers-0.3.0.0, because the darcs version is not
> > >buildable (Control/Monad/Signatures.hs is not in the repository).
> > >
> > 
> > [Sorry to revive this 1 yr 1/2 thread, but I was looking at the reason why
> > we don't have eitherT is a more canonical place like transformers,
> > and ended up here ...]
> > 
> > Is there a reason why this thread ran out of steam ? It not totally obvious
> > from the answers what was missing to move forward.
> > It seems like there's general agreement that EitherT is a good addition, did
> > this proposal got forgotten ? Is there a way to revive this proposal ?
> 
> IIRC, last time this was brought up, Ross said that he'd take action after GHC
> 7.8 is released. So your reminder is well timed, in fact ;)

Instead of EitherT, the next version will deprecate ErrorT in favour
of a transformer ExceptT with base monad Except.  The idea is to have
analogous transformers and monads

	ExceptT : Except : Either
	WriterT : Writer : (,)
	ReaderT : Reader : (->)

Other changes are:

        * Added infixr 9 `Compose` to match (.)
        * Added Eq, Ord, Read and Show instances where possible
        * Replaced record syntax for newtypes with separate inverse functions
        * Added delimited continuation functions to ContT
        * Added instance Alternative IO to ErrorT

Pre-release docs are here:

	http://code.haskell.org/~ross/transformers/dist/doc/html/transformers/