Proposal: merge either into transformers

[Index Int]

Perhaps ExceptT is what you're looking for? Check out the
'Control.Monad.Trans.Except' module in 'transformers'.

On Mon, Jan 2, 2017 at 4:24 AM, Erik de Castro Lopo
<mle+hs at mega-nerd.com> wrote:
> Vincent Hanquez wrote:
>
>> 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 ?
>
> So now its me reviving a 2 1/2 year old response to a 6 year old thread.
>
> I came here exactly the same way as Vincent, wanting to a couple of new
> combinators added to the either library, noticing that there was talk
> of moving Control.Monad.Trans.Either to transformers or transformers-compat
> and then it seemed to loose steam.
>
> Any clues as to where we're at with this?
>
> Erik
> --
> ----------------------------------------------------------------------
> Erik de Castro Lopo
> http://www.mega-nerd.com/
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