[Haskell-cafe] How to determine correct dependency versions for a library?

Clark Gaebel cgaebel at uwaterloo.ca
Wed Nov 14 21:21:27 CET 2012

To prevent this, I think the PVP should specify that if dependencies get a
major version bump, the package itself should bump its major version
(preferably the B field).

Hopefully, in the future, cabal would make a distinction between packages *
used* within another package (such as a hashmap exclusively used to
de-duplicate elements in lists) and packages *needed for the public
API*(such as Data.Vector needed for aeson). That way, internal
packages can
update dependencies with impunity, and we still get the major version
number bump of packages needed for the public API.

  - Clark

On Wed, Nov 14, 2012 at 2:20 PM, Tobias Müller <troplin at bluewin.ch> wrote:

> Peter Simons <simons at cryp.to> wrote:
> > Hi Clark.
> >
> >  > I think we just use dependencies [to specify] different things.
> >
> > If dependency version constraints are specified as a white-list --
> > i.e. we include only those few versions that have been actually
> > verified and exclude everything else --, then we take the risk of
> > excluding *too much*. There will be versions of the dependencies that
> > would work just fine with our package, but the Cabal file prevents
> > them from being used in the build.
> >
> > The opposite approach is to specify constraints as a black-list. This
> > means that we don't constrain our build inputs at all, unless we know
> > for a fact that some specific versions cannot be used to build our
> > package. In that case, we'll exclude exactly those versions, but
> > nothing else. In this approach, we risk excluding *too little*. There
> > will probably be versions of our dependencies that cannot be used to
> > build our package, but the Cabal file doesn't exclude them from being
> > used.
> >
> > Now, the black-list approach has a significant advantage. In current
> > versions of "cabal-install", it is possible for users to extend an
> > incomplete black-list by adding appropriate "--constraint" flags on
> > the command-line of the build. It is impossible, however, to extend an
> > incomplete white-list that way.
> >
> > In other words: build failures can be easily avoided if some package
> > specifies constraints that are too loose. Build failures caused by
> > version constraints that are too strict, however, can be fixed only by
> > editing the Cabal file.
> >
> > For this reason, dependency constraints in Cabal should rather be
> > underspecified than overspecified.
> The blacklisting approach has one major disadvantage that noone has
> mentioned yet:
> Adding more restrictive constraints does not work, the broken package will
> be on hackage forever, while adding a new version with relaxed constraints
> works well.
> Consider the following example:
> A build-depends: B ==2.5.* C ==3.7.* (overspecified)
> B build-depends: C ==3.* (underspecified)
> C
> Everything works nice until C- appears with incompatible changes
> that break B, but not A.
> Now both A and B have to update their dependencies and we have now:
> A build-depends: B ==2.5.* C >=3.7 && <3.9
> B build-depends: C >=3 && <3.8
> C
> And now the following combination is still valid:
> A
> B (old version)
> C
> Bang!
> Tobi
> PS: This is my first post on this list. I'm not actively using haskell, but
> following this list for quite a while just out of interest.
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