[asdf-devel] [cclan-list] How to depend on a system optionally?

Robert Goldman rpgoldman at sift.info
Mon Jul 20 23:41:19 UTC 2009

james anderson wrote:
> On 2009-07-21, at 01:18 , Robert Goldman wrote:
>> james anderson wrote:
>>> On 2009-07-21, at 00:35 , Robert Goldman wrote:
>   [...]
>>>>   What would it mean, for
>>>> example, to be :contingent-on or :weakly-depend on a :file?
>>> if the file is present, then the contingent component is included in
>>> the system and dependent on that file.
>>> no different than a simple dependency. the only difference are in the
>>> effective behaviour of operate.
>>> in my case it makes it possible to do things like include a ffi
>>> interface iff dependent on runtime conditionalization and include a
>>> particular aspect of the higher level interface iff the particular
>>> ffi interface is there.
>> I don't believe that is correct.
> ?
> in that weakly-depends-on does not do this, or this is not a  
> "correct" thing to do?

The former.  weakly-depends-on does not do these sophisticated things.

> i do not use weakly-depends-on. i implemented :contingent-on.
>>   If you look at the source,
>> weakly-depends-on is not handled as a first class citizen, the way
>> :depends-on is.  :depends-on is squirreled away in a slot, and then
>> manipulated at the time we carry out an ASDF operation.  The WEAKLY  
>> part
>> of :weakly-depends-on, on the other hand (1) only works when the
>> dependency is on a system, not a feature or a file and (2) is resolved
>> when the defsystem is "parsed".  I.e., there's no way you can load a
>> system that has weakly-depends on, THEN define another system on which
>> it weakly depends, and have the first system do anything reasonable.
>> The :weakly-depends-on is gone by the time the second system is
>> introduced to your lisp image.
> weakly depends on may well not be well implemented. i do not use it.

Yes, that's my point.  I'd rather it not become widely used, lest it
become another oddity with which we must maintain backward compatibility.

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