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
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