[Haskell-cafe] Re: Proposal: register a package as providing
several API versions
simonmarhaskell at gmail.com
Tue Oct 16 08:57:01 EDT 2007
> 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
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