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