[Haskell-cafe] The applicative instances for Either?

[Paterson, Ross]

Ketil Malde writes:
> I have a program that makes use of the applicative instance for Either
> String.  I used to define these instances locally, but at some point,
> they became part of Control.Applicative.  I have limited the
> dependencies to 'base >= 4', but apparently, some version 4s of base
> include this instance, some do not, and it causes problems for people
> trying to compile the program.

> Is there any information, or otherwise accessible source specifying
> exactly when this was changed, so that I can have more precise
> dependencies?

base is released with GHC, so the info is in the release notes for a
major GHC release (library interfaces aren't allowed to change in
minor releases):

http://www.haskell.org/ghc/docs/7.0.1/html/users_guide/release-7-0-1.html#id3037360

1.5.10.2. base
* Version number 4.3.0.0 (was 4.2.0.2) 
[...]
* There are now Applicative, Monad and MonadFix instances for Either.