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

[Tobias Müller]

Peter Simons <simons at cryp.to> wrote:
> Hi Tobias,
> 
>  > A 1.1.4.0 build-depends: B ==2.5.* C ==3.7.* (overspecified)
>  > B 2.5.3.0 build-depends: C ==3.* (underspecified)
>  > C 3.7.1.0
>  >
>  > Everything works nice until C-3.8.0.0 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 1.1.5.0 build-depends: B ==2.5.* C >=3.7 && <3.9
>  > B 2.5.4.0 build-depends: C >=3 && <3.8
>  > C 3.8.0.0
>  >
>  > And now the following combination is still valid:
>  > A 1.1.5.0
>  > B 2.5.3.0 (old version)
>  > C 3.8.0.0
>  > Bang!
> 
> thank you for contributing this insightful example.
> 
> When such a situation has arisen in the past, it's my experience that the
> author of B typically releases an update to fix the issue with the latest
> version of C:
> 
>   B 2.5.4.0 build-depends: C >= 3.8
> 
> So that particular conflict does hardly ever occur in practice.

And what if the maintainer of a takes the chance to make some major updates
and directly releases 2.6? Then all packages depending on 2.5.* will
probably break. 

> Note that package A would build just fine after that update of B -- if the
> author of A hadn't overspecified its dependencies. As it is, however, a
> new version of A has to released that changes no code, but only the Cabal
> file.

But all this boils down to a system where only a combination of latest
versions will be stable. So why restrict dependencies anyway?

Tobi