Proposal: register a package as providing several API versions

[Daniel McAllansmith]

On Wednesday 17 October 2007 01:32, 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.

If 3.0.0 is a superset of 2.1.1 why was it necessary to bump to 3.0.0?  Why 
not 2.2.0?