Hi, I proposed multiway flags about a year ago [1]. With that you could write: flag xx values: a, b, c, d if flag(xx == a) build-depends: xx-a if flag(xx == b) build-depends: xx-b if flag(xx == c) build-depends: xx-c if flag(xx == d) build-depends: xx-d The benefits over your proposal, is that there's only one flag! The alternatives mentioned in the issue [1] consider only automatic flags, but now as I think about, manual multiway flags can be useful too. --- For complex "multiway" selection example see: http://hackage.haskell.org/package/functor-classes-compat-1/functor-classes-... The idea is that with two flags (transformers-1 and transformers-2) you have already 4 combinations, so you don't need 4 flags. That's not intuitive interface for manual flags though. (There the "elif" syntax would help, which I'll add to Cabal for GHC-8.4) --- Another way to write "mutex" is (with three flags: a, b, c): -- at least one: if !(flag(a) || flag(b) || flag(c)) -- at most one: if flag(a) && flag(b) build-depends: base<0 if flag(a) && flag(c) build-depends: base<0 if flag(b) && flag(c) build-depends: base<0 That's tedious to go thru all pairs, but quite straightforward. And can live in a separate block from where the actual flag-logic is defined. The same trick can be used with two-flags for three options: if flag(xx-1) if flag(xx-2) -- option 1 else -- option 2 else if flag(xx-2) -- option 3 else build-depends: base<0 It's also discussed to add explicit fail: Unsupported configuration so we don't need to use unsatisfiable "base<0" constraint. - Oleg [1]: https://github.com/haskell/cabal/issues/3526 On 19.07.2017 04:50, Lee John wrote:
I think the mutex flags might be needed. For example, if my program use accelerate and I want my program can use both of cuda, llvm, (etc) as the backend for accelerating, I need to write a lot of if-statements in cabal file to control that there will be only one backend to be used. So I think that mutex-flags might be useful.
For example:
… flag a default: False flag b default: False flag c default: False flag d default: False ... exec… … if flag(a) && !flag(b) && !flag(c) && !flag(d) build-depends: xx-a
… if !flag(a) && flag(b) && !flag(c) && !flag(d) build-depends: xx-b
… if !flag(a) && !flag(b) && flag(c) && !flag(d) build-depends: xx-c
… if !flag(a) && !flag(b) && !flag(c) build-depends: xx-d …
The above codes are also hard to maintain or extend. Following codes will better.
… flag a default: False flag b default: False flag c default: False flag d default: False mutex flags: a b c d default: d …
if flag(a) build-depends: xx-a
if flag(b) build-depends: xx-b
if flag(c) build-depends: xx-c
if flag(d) build-depends: xx-d ... _______________________________________________ cabal-devel mailing list cabal-devel@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/cabal-devel