Re: [Haskell-cafe] The applicative instances for Either?

A natural reference would be the darcs repository http://darcs.haskell.org/packages/base/ or its git mirror http://darcs.haskell.org/packages/base.git which I used because getting it is much faster. Using 'git blame Control/Applicative.hs' I figured out that the instance was added in this commit: commit 8abb469bd2210d78da74b334a0f4397be5ac37f6 Author: Ross Paterson Date: Thu Jun 17 22:51:10 2010 +0000 add Applicative instance for Either (proposal #4095) This is not the only possible instance for Either, but this one is compatible with the usual Monad instance. Unfortunately, the git repository doesn't have proper tags that correspond to the package version (if it did, we'd use 'git describe'). So, just go to the hackage page and check which release was the first one after Jun 17 2010. It was 4.3.0.0, released on Nov 16 2010. And so your dependency might look like "base >= 4.3". OTOH, it would be very nice of library developers to have changelogs and/or proper version tags in the VCS. * Ketil Malde [2011-09-17 08:42:42+0200]

Hi,

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? And is there a simple way to handle this conditionally, either within cabal, or using CPP?

-- Roman I. Cheplyaka :: http://ro-che.info/