Proposal: register a package as providing several API versions

[ChrisK]

Simon Marlow wrote:
> Several good points have been raised in this thread, and while I might
> not agree with everything, I think we can all agree on the goal: things
> shouldn't break so often.

I have another concrete proposal to avoid things breaking so often.  Let us
steal from something that works: shared library versioning on unixy systems.

On Max OS X, I note that I have, in /usr/lib:
> lrwxr-xr-x    1 root  wheel        15 Jul 24  2005 libcurl.2.dylib -> libcurl.3.dylib
> lrwxr-xr-x    1 root  wheel        15 Jul 24  2005 libcurl.3.0.0.dylib -> libcurl.3.dylib
> -rwxr-xr-x    1 root  wheel    201156 Aug 17 17:14 libcurl.3.dylib
> lrwxr-xr-x    1 root  wheel        15 Jul 24  2005 libcurl.dylib -> libcurl.3.dylib

The above declaratively expresses that libcurl-3.3.0 provides the version 3 API
and the version 2 API.

This is the capability that should be added to Haskell library packages.

Right now a library can only declare a single version number.  So if I update
hsFoo from 2.1.1 to 3.0.0 then I cannot express whether or not the version 3 API
is a superset of (backward compatible with) the version 2 API.

Once it is possible to have cabal register the hsFoo-3.0.0 also as hsFoo-2 it
will be easy to upgrade to hsFoo.  No old programs will fail to compile.

Who here knows enough about the ghc-pkg database to say how easy or hard this
would be?

-- 
Chris