On Saturday 17 September 2011, 08:42:42, Ketil Malde wrote:
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,
Commited on 18th June 2010 to HEAD ;) Checking the sources, it wasn't in base-4.2.0.2 (ghc-6.12.3), but it was in base-4.3.1.0 (ghc-7.0.2), so it was introduced with base-4.3
so that I can have more precise dependencies? And is there a simple way to handle this conditionally, either within cabal, or using CPP?
Simplest way would be CPP, I think. Cabal provides a MIN_VERSION_foo macro, so -- The extra parentheses might be necessary, some Cabal version(s) -- had a buggy MIN_VERSION macro, I don't remember the details #if !(MIN_VERSION_base(4,3,0)) instance Applicative (Either e) where ... #endif You can have a flag based on which the version of the module with or without the instance is imported, but the above seems much simpler