Imagine we have two packages p and q that are normally independent, i.e. one can link p without q, and q without p. However, when both p and q are used in the same program, it is expected that they cooperate with each other. Of course, this situation can be modeled with the help of subpackages (a real-world example of this are p=lablgtk and q=lablgl).
The idea is as follows: p has a subpackage p.for_q that contains code with special features for q, and q has a subpackage q.use_p that depends on p, p.for_q, and q, and that contains the code using the special features of p. Expressed in META files, p would define
# META file for p: requires = "..." archive = "..." package "for_q" ( requires = "p" archive = "..." )and q would define
# META file for q: requires = "..." archive = "..." package "use_p" ( requires = "q,p.for_q" archive = "..." )Of couse, the program using both libraries must explicitly enable the subpackages by mentioning "q.use_p" on the ocamlfind command line, otherwise the glue code would be omitted.
Unfortunately, it is not possible to use the package predicates pkg_p and pkg_q to add archives depending on whether the other package is also selected. The problem is that the order cannot be specified, i.e. whether p must be linked first or q.