1- ASDF has no such mechanism yet. Many people have asked for it (at least nikodemus, manic12, and myself recently). 2- the feature feature was never working before I fixed and cerror'ed it, and only does conditional *dependency* on another component, that has to independently exist or conditionally exist, for which we have no non-ugly mechanism. 3- the :if-component-dep-fails mechanism may or may not be working well enough to do that right now, but man it's ugly, non-intuitive, non-documented, and gross even if it were documented. 4- the very structure of ASDF objects doesn't allow a clean way of expressing conditionally-included components, and this can't be added as an extension (at least, not unless ASDF is extended to provide a proper API, anyway). Doing anything more sophisticated than #+foo or :if-component-dep-fails is going to be "interesting" and require major work. 5- XCVB has such a mechanism, and I propose that if ASDF is to adopt non-trivial conditionals, we should import XCVB's mini-language for conditionals, if not the rest of it. 6- Hey - why not develop XCVB some more? Like, port it to your favorite platform? I'd need a bit of help to make standalone backends for Windows, including a committed hacker to port relevant parts of IOLib to Windows.
[ François-René ÐVB Rideau | Reflection&Cybernethics | http://fare.tunes.org ] Invent a clever saying, and your name shall live forever. — Anonymous