[asdf-devel] Ordering of system :depends-on elements

[Robert Goldman]

Zach Beane wrote:
> Robert Goldman <rpgoldman at sift.info> writes:
> 
>> Zach Beane wrote:
>>> A few projects in quicklisp work something like this:
>>>
>>>    ;;; foo.asd
>>>
>>>    (defsystem foo ...)
>>>
>>>    (defsystem foo-extra ...)
>>>
>>>
>>>    ;;; bar.asd
>>>
>>>    (defsystem bar :depends-on (:foo-extra :foo))
>>>
>>> With asdf 2, (asdf:load-system "bar") seems to work fine, I guess
>>> because asdf 2 does the equivalent of find-system on the elements from
>>> right-to-left.
>>>
>>> With asdf 3, it doesn't seem to work fine, I guess because asdf 3 does
>>> the equivalent of find-system on the elements from left-to-right.
>>>
>>> Are those guesses correct?
>>>
>>> What's the best way to have a system definition that works equally well
>>> in asdf2 and asdf3 in this kind of situation?
>>
>> I feel like I'm missing something.  Is there some reason you can't simply make 
>>
>> (defsystem foo-extra :depends-on (:foo)
>>   ....)
>>
>> ?
> 
> If they were in separate files, yes. They aren't.

I misunderstood your original question:  I thought that this was a
dependency problem, but it's not.  The problem is that the system
authors depended on something contingent about the way FIND-SYSTEM was
invoked to find depended-upon systems.  I haven't gone back to check,
but my guess is that something once iterated and pushed, and now collects.

> This was relied upon by some projects, and now it doesn't work.

I'm sorry, but to the best of my knowledge that behavior was never
offered up by ASDF to be relied upon.  Programmers  should not have
depended on this behavior.

The only solution I know of is to break foo.asd into multiple files.  I
realize that this is inconvenient, but restoring backward compatibility
involves restoring a state where programmers are relying on a reading
that is not just contingent, but contingent in a way that encourages
people to write system definitions in a way that will be actively
misleading to the reader (since conventionally we read left to right).

Does this happen in enough systems in quicklisp that this is a serious
problem?  How many?  Perhaps we can work out some solution.  I realize
that in many cases the one-system-per-file mapping can be a nuisance.

Best,
r