Proposal to solve the `EitherT` problem.

[MightyByte]

For me, the name EitherT wins by a large margin.  Way back when I
first learned about MaybeT, it totally rocked my world.  I had been
plagued for quite some time by the problems that MaybeT addresses but
didn't know of the solution.  I had already seen things like ReaderT
and StateT, but for some reason I hadn't yet made the connection to
how the idea would carry over to the Maybe monad.  After I learned
about MaybeT, EitherT was the natural name I expected.  If EitherT had
been in transformers, I think I would have used it even sooner.

I think the discoverability of the name EitherT is huge win given the
existing conventions.  I think I had even seen ErrorT before MaybeT,
but the name obscured its significance for me.

Also, as Ed pointed out, I do use the errors package in snap.  These
days I use errors in pretty much every substantial project I work on.
Usually that includes EitherT, so renaming it would impose a huge
cost.



On Tue, Aug 13, 2013 at 4:43 PM, Dag Odenhall <dag.odenhall at gmail.com> wrote:
> Also, grepping hackage is all well and good but that‘s ignoring all the code
> that’s either not released yet or is closed source. That's probably the case
> with a lot of application code, with hackage having mostly libraries. Just a
> thought.
>
>
>
> On Tue, Aug 13, 2013 at 10:39 PM, Edward Kmett <ekmett at gmail.com> wrote:
>>
>> Did you consider the transitive dependency from errors?
>>
>> Errors re-exports Control.Monad.Trans.Either from Control.Error.
>>
>> In particular I noted that snap then depends on errors.
>>
>> I didn't grep through the source though.
>>
>> -Edward
>>
>>
>> On Tue, Aug 13, 2013 at 1:25 PM, Ross Paterson <R.Paterson at city.ac.uk>
>> wrote:
>>>
>>> On Tue, Aug 13, 2013 at 10:30:23AM -0400, Edward Kmett wrote:
>>> > An argument against just randomly bikeshedding the name it is there
>>> > are a lot of packages out there currently transitively depending on
>>> > the existing either package, due to the popularity of Tekmo's errors
>>> > package and the fact that it has been picked up by snap. So half of
>>> > the web-apps in the ecosystem depend on this type transitively.
>>>
>>> Fortunately it seems that EitherT is only used by the following packages:
>>>
>>>         citation-resolve coroutine-object CSPM-Frontend errors
>>>         happstack-heist hoodle-core hoodle-parser katt pdf-toolbox-core
>>>         pianola restricted-workers terminfo-hs
>>>
>>> Moreover adding a new module and type means people can switch over an
>>> extended timescale.  Thus I think internal consistency within
>>> transformers
>>> outweighs compatibility with the existing EitherT in this case.
>>>
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>>
>>
>>
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