With the current issue of the split base library we have a lot of code like: flag split-base library if flag(split-base) build-depends: base >= 3.0, pretty, directory, etc else build-depends: base < 3.0 This does a backtracking search, checking both alternatives to see if one is satisfiable. It's a bit unsatisfactory however since we have to introduce a flag for a choice which really should be automatic and not visible to the user. It was always a deliberate part of the design of configurations that we should not let package authors make automatic decisions without the environment (packager or user) being able to influence the choice. So the question is, is there a special case that we can say really is an automatic decision that the user does not need to control with a flag? 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) So we introduce a new predicate 'package()' that tests the version number of a package that we've already declared that we depend on. It is crucial that it be one we already depend upon. It does not test packages in the environment generally, it tests the version of the package that the environment has or might impose. So we still introduce a backtracking point, it's just that now it's an 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. Details... each package predicate test introduces it's own distinct anonymous flag. The flags are introduced in order, top to bottom, left to right and appear after all other named flags. eg: flag blah build-depends: foo, bar if package(foo > 1) && package (bar > 1) && flag(blah) ... else ... => flag blah flag _1 flag _2 build-depends: foo, bar if flag(_1) && flag(_2) && flag(blah) build-depends: foo > 1 build-depends: bar > 1 ... else build-depends: !(foo > 1) build-depends: !(bar > 1) ... Saying that we already depend on a package is interpreted relative to the position of the package() test in the overall condition tree. It means that to get to the branch where the package() test is, we must have that package in the build-depends explicitly in some parent node. Possibly as an extension we could allow it to appear in all the alternatives of some sibling node eg: So, overall, does that make sense? Is that sufficiently precise? Should we do it? Here's the canonical example. It's a package (tar) that works with ghc-6.4, 6.6 and 6.8 and uses bytestring: Flag bytestring-in-base Flag split-base library if flag(bytestring-in-base) -- bytestring was in base-2.0 and 2.1.1 build-depends: base >= 2.0 && < 2.2 else -- in base 1.0 and 3.0 bytestring is a separate package build-depends: base < 2.0 || >= 3, bytestring >= 0.9 if flag(split-base) build-depends: base >= 3.0, directory, old-time else build-depends: base < 3.0 which we could simplify to: library build-depends: base if package(base >= 3) directory, old-time if package(base < 2 || >= 3) build-depends: bytestring >= 0.9 Much nicer I think. Duncan