import resolution (was RE: exposed packages and cabal depends)
S. Alexander Jacobson
alex at alexjacobson.com
Fri Apr 15 18:45:27 EDT 2005
On Thu, 14 Apr 2005, Simon Peyton-Jones wrote:
> The Right Thing to
> do must be to regard the "full module name" as being the package name
> pre-pended to the module name.
I don't think that is right. As far as I understand, module names in
import statements identify specifications not implementations.
Otherwise, it would be incorrect for different compilers to implement
the Prelude differently. Resolution of imported module names to
meanings or specifications is fundamentally different from resolution
of module names to implementations. The former is the domain of the
programmer and the later is the domain of the compiler/interpreter.
> b) We worry about error messages like "cannot unify M.T with M.T"
> (meaning different M's!)
I think the Haskell report is pretty clear that M.T always means M.T.
So, I interpret this as an error in the status quo that can result
from multiple packages requiring conflicting implementations of M.
The core problem here is that packages should not be *requiring*
implementations of anything. They should only be providing them.
Under the package overlap restriction, containment is identical with
requirement. So the pacakge overlap restriction is a minor instance
of this problem A much more major instance results from putting
build-depends tags in cabal files. Instead of being unable to use
package A with package B if both *contain* an implementation of
Foo.Baz, build-depends means that you are unable to use package A with
package B if both *require* an implementation of Foo.Baz. The
likelihood of conflict therefore rises considerably.
What we really need here is Hackage on steroids; a way for
compilers/interpreters to look up module names in shared
directories/name-servers in order to obtain packaged implementations
that are compatible with the target platform (perhaps via a compile)
and with each other (if the package overlap restriction survives).
I know we've been going back and forth on this discussion for a while
and my position has changed some. I want to thank you for your
patience. I hope this post finally makese some sense.
* I think we should also have a somewhat more explicit system for
allocating module namespace, but that is a whole separate discussion
whose resolution is dependent on this one.
PS Though I didn't quote them, I think this post also addresses the
concerns expressed in David Roundy, Wolfgang Thaller, Simon Marlow's
and Isaac Jones. If not, please advice.
S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com
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