[Haskell-cafe] Cabal and more than one version

[Duncan Coutts]

On Wed, 2008-11-19 at 02:15 -0800, Jason Dusek wrote:
> Duncan Coutts <duncan.coutts at worc.ox.ac.uk> wrote:
> > Jason Dusek wrote:
> > > In the ticket, someone says:
> > >
> > >  True though I suspect it looks a bit weird to the
> > >  uninitiated. We know to read the conditional syntax as an
> > >  implication constraint which can be applied in either
> > >  direction but I suspect many people read it in a directed
> > >  functional way.
> > >
> > > Does that mean you don't have to actually set 'splitBase'
> > > explicitly?
> >
> > No, that is the point. You do not have to. The choice for the
> > flag is completely determined automatically by the version of
> > the base package that is chosen.
> 
>   When you say "chosen" is that the same as "detected"?

No. Each package can only be built against a single version of a
dependent package. So chosen really means chosen. It is the single
version of the dependent package that we end up building your package
against.

>  With 6.10, both versions of base are available -- what happens
>   then? The environment chooses the latest one?

Good example. So in that case the system (with optional input from the
user / package manager) has to choose one of them. Often there will be a
constraint from the package so that there will be no real choice. When
the choice is not constrained then it is more complicated:

      * 'rughc Setup configure' and 'cabal configure' use a simple (and
        occasionally wrong) heuristic to pick the latest version.
      * 'cabal install' uses a simple constraint solver and some global
        and local preferences to make the choice. At the moment the
        global preference is to pick base 3, because many many packages
        say "build-depends: base >= 3" but in fact fail when built with
        base-4. So the preference is a kind of patch to keep them
        building.

Duncan