: Faré : Robert
I strongly recommend against erroring in the .asd file itself based on the implementation, because that hinders cross-compilation.
IIUC what this boils down to is a preference for failing to *operate* on the system, rather than failing to parse the system definition. Correct?
If we ever want to support cross-compilation, it's more than a preference: it's a bug to fail to load an .asd file just because the system won't load on the current (master) implementation — it is meant to work on the (slave) cross-implementation.
[As an aside, there may be a separate bug: when I tried to load DecLt into ACL, the loading *succeeded*, but the contents of the decLt "core" subsystem failed to be loaded. So I believe there may be an oddity in the way ASDF is handling these "failed" :feature dependencies. I will try to replicate and provide a minimal test case.]
I'd say it's a bug in DecLt rather than ASDF. It's :if-feature :sbcl says that the system is a NOP when the target implementation isn't sbcl. If what is intended instead is that the system should error out when the implementation isn't sbcl, it should use something like :depends-on (... (:feature (:not :sbcl) "implementation-not-supported") ...) or to be cleaner: :components ((:file "implementation-not-supported" :if-feature (:not :sbcl)))
The traditional solution is to load a file that fails. i.e. create a file unsupported-implementation.lisp that has an (error ...) form and in your asd file, use a component (:file "unsupported-implementation" :if-feature (:not :sbcl))
I would prefer that we provide a more readable version of this. I think we could readily add
(:depends-on (:essential-feature :sbcl))
I don't like that name, and I don't like abusing :depends-on for it. If you want an error when compiling outside a given set of implementations, maybe it's better having a defsystem option. it doesn't make sense at the component level, so let's not hack the depends-on language for it. (defsystem declt ... :error-if-not-feature :sbcl ...) :(fail|error)-(if-not|unless)(|-feature) :only-build-(on|with-feature) :demand-feature
As I said earlier, the "bogus file" solution seems to me unpleasantly indirect, because then the poor reader must grovel over "unsupported-implementation" to see what will happen if one tries to load it, when the intent is simply to say "this won't work unless you are on SBCL."
That's quite a reasonable feature request. I was only describing the current "best practice".
I believe we could provide this with the semantics you propose (fail on operation, not on loading the system definition).
You could fail while building the plan, rather than performing it. Building a plan is already feature-dependent. That would save users a lot of patiently waiting for dependencies to build until they get their doomed answer. —♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org Live as if you were living already for the second time and as if you had acted the first time as wrongly as you are about to act now! — Viktor Frankl, "Man's Search for Meaning"