Proposal: merge either into transformers

Erik de Castro Lopo mle+hs at
Mon Jan 2 01:24:43 UTC 2017

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-, 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 de Castro Lopo

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