Re: [Haskell-cafe] Cabal and more than one version

20 Nov 2008


      On Wed, 2008-11-19 at 02:15 -0800, Jason Dusek wrote:
...
Duncan Coutts  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