Add --numeric-version as default

[pjonsson]

HLint is my use case for this feature request, but I think the feature is general enough to sit in command line parsing libraries.

Cabal supports the flag --numeric-version which is convenient for version detection/comparison in scripts/tools. I looked around in the source but I don't see any obvious ways of adding this flag without breaking the current API.

Can you either sketch how to cause the least breakage and I can prepare a patch, or add support for the flag?

[ndmitchell]

The feature doesn't need adding to the Explicit stuff at all, since there you code up your own --version string. For the Implicit stuff I would add cmdArgsNumericVersion to that type, then follow the pattern of summary to add a new numericVersion annotation. Then people can write &= numericVersion "0.0.4" and their app will support--numeric-version`. I don't foresee that causing any breakage.

[pjonsson]

Do you see any way to encourage people to add that annotation, besides documenting it properly?

[ndmitchell]

The alternative is to take the summary annotation and look for things that look plausibly version shaped. I'd be happy with that.

[ndmitchell]

Added, and I checked it works with HLint.