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

[james anderson]

On 2009-07-21, at 00:58 , Robert Goldman wrote:

> Gary King wrote:
>> I share Robert's queasiness and also think that we want ASDF to  
>> support
>> these sorts of dependencies (simple, weak, contingent, etc... (?)).
>>
>> I'm going to update the manual with James's table (thanks James!)  
>> in the
>> hopes of giving us a place to hang our collective hats.
>>
>>
>
> [...]
>
> Follow-on suggestion:  we need to think about what happens when a
> component has all possible combinations of :depends-on,
> :weakly-depends-on, and :contingent-on.  Does something that would not
> be loaded because of :contingent-on avoid a crash that would come  
> from a
> violated :depends-on?

hmmm... depends on what you mean by a "crash".
it probably does not make sense to define a relation like

(:file "a" (:contingent-on :osx))
(:file "b" (:depends-on "a"))

as that reads as if that "a" will always be present, which is likely  
to cause confusion if the statements are not _real_ close neighbors  
in the system description.

what does make sense is

(:module "x"
   (:components ((:module :ffi
                    (:components ((:file "a-ffi" (:contingent-on  
"osx" :ccl))
                                  (:file "a-inter" (:contingent-on "a- 
ffi"))
                                  (:file "b-ffi" (:contingent-on  
"linux" :sbcl))
                                  (:file "b-inter" (:contingent-on "b- 
ffi")))))
                 (:file "x-ops" (:depends-on :ffi)))
>
> Extra bonus follow-on:  are these expected to be meaningful when the
> depended-on item is a system (clearly yes), a feature (seems  
> handy), an
> arbitrary component (what would this mean?)?

i have used contingent dependencies for all of them.

eg. if a graphics system is to be built somewhere where clx makes  
sense, then there is a system dependency between the graphics package  
and clx.
in addition, there is a file-level dependency between the clx-based  
implementation of rendering operators and clx's presence.

one _can_ express everything in terms of #+, but not as clearly.