[Haskell-cafe] The applicative instances for Either?

Paterson, Ross R.Paterson at city.ac.uk
Sat Sep 17 12:21:55 CEST 2011

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 base
* Version number (was 
* There are now Applicative, Monad and MonadFix instances for Either. 

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