On Tue, Oct 16, 2007 at 11:15:14PM +0100, Duncan Coutts wrote:
Here's my suggestion:
library build-depends: base if package(base >= 3) build-depends: pretty, directory, etc
and it is syntactic sugar for:
flag _unnamed1
library build-depends: base if flag(_unnamed1) build-depends: base >= 3.0 build-depends: pretty, directory, etc else build-depends: !(base >= 3.0)
Your examples use base, for which there will be only one version in a GHC installation. But there might be multiple versions of other packages, so testing on them is a bit more complicated. How does a repackager control the selection? (On the other hand, it may be that flags are to coarse a control for them anyway; they might want to specify versions or version ranges too.)
So we still introduce a backtracking point, it's just that now it's backtracking without a named flag. This is ok because the decision about which branch to take is completely determined once the environment picks a version for the dependent package in question.
If this is just sugar for flags, won't it still backtrack just as much? Also, when one branches on a flag, the first alternative is preferred if both work. Presumably this will still be the case, so one might sometimes have to write an inverted test with an empty then-branch.