Re: [Haskell-cafe] Alternative dependencies in Cabal file

17 Mar 2010


      Thanks, I missed that the flags are set dynamically if a dependency 
cannot be satisfied.


Am 17.03.2010 13:23 schrieb Daniel Fischer:
...
Am Mittwoch 17 März 2010 12:11:53 schrieb Matthias Reisner:
...
Hi,
for a package I need to ensure the user uses a certain package
configuration. So how would I rewrite the following pseudo-cabal
description?
Build-Depends: packageA < X,           packageB < Y
                     or
                   packageA >= X && < X',  packageB >= Y && < Y'
                     or
                   packageA >= X',         packageB >= Y'
Build-Depends: ... common dependencies ...
where neither A nor B is the base package. Maybe I have to use if/else
blocks, but I don't know what conditions to use then.
Read http://www.haskell.org/ghc/docs/latest/html/Cabal/authors.html for a 
general description of what you can do, I'd try something like in
http://hackage.haskell.org/packages/archive/cabal-install/0.8.0/cabal-
install.cabal
flag oldAB
    description: ancient packages A and B
    default: False
flag newAB
    description: shiny new A and B
Library blubb
    build-depends:
        common,
        libraries
    if flag(newAB)
        build-depends: packageA >= X', packageB >= Y'
    else if flag(oldAB)
        build-depends: packageA < X, packageB < Y
    else
        build-depends: packageA >= X && < X', packageB >= Y && < Y'
If I remember correctly, that tries first to build against the new A and B, 
that failing, it sets flag newAB to false and tries again, first with the 
not-so-ancient A and B, hopefully (but I'm not sure about the order in 
which flags are toggled if the defaults don't give a successful install 
plan).
...
Regards,
Matthias