Re: asdf and provide / require

Gary King <[email protected]> writes:

> I'd like to build a system A that loads certain files if and only if  
> some other system B is _not_ loaded. One natural way to do this would  
> be to have system B push some feature (call it :b) onto *features* and  
> have system A say something like
>
> :components ((:file "something-like-b" :depends-on ((feature  
> (not :b)))))
>
> which is supposed to say "compile and load the file 'something-like-b'  
> if the feature :b is not present when you actually get around to  
> loading this system. Depends-on isn't quite the right term but I'm  
> hoping to keep changes down to a minimum.
>
> The above won't work both because ASDF can't express anything about  
> the absence of a feature and because you can't depend on a feature  
> being present in the depends-on clause. The closest I think you can  
> get now is
>
> :in-order-to ((:compile-op (feature :foo)) (load-op (feature :bar))))
>
> Is there some other to express what I want? Is there some reason that  
> I shouldn't be trying to express this at all!
>
> (Lastly, if there is any sense to this "include file iff some feature  
> is present / absent, then why not generalize and allow
>
> ;; more complex feature tests
> (:file "some-file" :include-when ((feature (:and :foo (:not :bar))))
>
> ;; more general computation
> (:file "some-file" :include-when ((test (some-predicate-i-know-not- 
> what-p)))
>
> (In the last psuedo-example, I'm imaging that the current component  
> and system are available via specials.)
>
> thoughts and comments welcome,

I think I came across this once before:

http://article.gmane.org/gmane.lisp.cclan.general/583

(Earlier posts in that thread that are mostly me being mislead by the
asdf manual at the time.)

What does feature-testing have to do with REQUIRE and PROVIDE (which
deal with *MODULES*, not *FEATURES*)?

--
Richard


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