Proposal: register a package as providing several API versions

[Simon Marlow]

ChrisK wrote:
> 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.

Certainly, this is something we want to support.  However, there's an 
important difference between shared-library linking and Haskell: in 
Haskell, a superset of an API is not backwards-compatible, because it has 
the potential to cause new name clashes.

> 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?

It could be done using the tricks that Claus just posted and I followed up 
on.  You'd need a separate package for hsFoo-2 that specifies exactly which 
bits of hsFoo-3 are re-exported.  Given some Cabal support and a little 
extension in GHC, this could be made relatively painless for the library 
maintainer.

Cheers,
	Simon