Module reexports at the package level

Roman Cheplyaka roma
Fri Oct 4 12:43:08 UTC 2013

* Joachim Breitner <mail at> [2013-10-04 12:33:43+0200]
> Hi,
> Am Freitag, den 04.10.2013, 13:05 +0300 schrieb Roman Cheplyaka:
> > I have two concerns about versioning:
> > 
> > 1. Suppose that package-a re-exports Data.Bar from package-b starting
> >    from package-a-2.0 and package-b-2.0. This means that we shouldn't
> >    prevent packages to be built with package-a-1.0 and package-b-2.0,
> >    because that would result in a duplicate module Data.Bar.
> you mean ?should?, right?

Yes, sorry.

> I?m not sure if I understand your example. Does package-a-1.0 provide
> Data.Bar? Are you sure that this is not covered by the PVP?

The problematic situation could look like this:


  provides Data.Bar


  provides Data.Bar


  re-exports Data.Bar from package-b-2.0

Now, the change between package-a-1.0 and package-a-1.0.1 may seem
almost invisible for the users, but if something depends on

  package-a ==1.0.*,
  package-b ==2.0

... then we have a conflict in case package-a-1.0 is picked.

But now I think this example is contrived because the constraint should
really be package-a >= 1.0.1 && < 1.1 (we could only test our
package successfully with package-a-1.0.1, and even ignoring module
re-exports, package-a-1.0.1 could have added new entities).

> > 2. If package-b has a major API change, it would bump its version, say,
> >    to 3.0. package-a still has version 2.0, but now it re-exports a
> >    completely different API.
> Good point. But package-a depends on package-b, and hence a major API
> bump in package-b will require at least a new minor version of
> packaga-a. It is then the obligation of the author of package-a to check
> if the API changes affect the re-exported packages (and I hope that
> eventually we?ll have tools that help with that).

You are right again ? I missed the fact that dependency package-a ->
package-b itself has a version constraint.

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