On 28/08/2008, at 23:59, Simon Marlow wrote:
The important thing about Cabal's way of specifying dependencies is that they can be made sound with not much difficulty. If I say that my package depends on base==3.0 and network==1.0, then I can guarantee that as long as those dependencies are present then my package will build. ("but but but..." I hear you say - don't touch that keyboard yet!)
Suppose you used autoconf tests instead. You might happen to know that Network.Socket.blah was added at some point and write a test for that, but alas if you didn't also write a test for Network.Socket.foo (which your code uses but ends up getting removed in network-1.1) then your code breaks. Autoconf doesn't help you make your configuration sound, and you get no prior guarantee that your code will build.
Cabal doesn't give this guarantee, either, since it allows you to depend on just network or on network>x. To be perfectly honest, I think neither autoconf's approach (free-form feature tests) nor Cabal's (version-based dependencies) really work for all important use cases. And I have to disagree with what you write below - I think both systems are fundamentally flawed. As I said before, what does (mostly) work IMO is depending on interfaces which are independent of packages. Being required to specify the exact interface you depend on solves the problem you describe above. It also solves the problem of name clashes with functions defined in later versions of a package. And it is still nicely declarative.
Both systems are flawed, but neither fundamentally. For Cabal I think it would be interesting to look into using more precise dependencies (module.identifier::type, rather than package-version) and have them auto-generated. But this has difficult implications: implementing cabal-install's installation plans becomes much harder, for example.
Interesting. From our previous discussion I got the impression that you wouldn't like something like this. :-) Roman