Expressing joint behavior of modules

Sometimes, special code is needed to make two modules interact together. This seems to come up all the time in Factor. For example, there are two language features, locals and prettyprinting, implemented in the Factor library. If they are both loaded, then there is special code to make locals prettyprint. But neither one depends on the other.

The old way to handle this situation was to have code that looked like this, as a top-level form in the locals vocab:

"prettyprint" vocab [
    "locals.prettyprint" require
] when

But this is problematic. What if locals is loaded first, and then prettyprint? Well, this actually happened, due to a change in some other code. Then locals don't prettyprint correctly! You could fix this by adding top-level statements in both vocabs, as mirrors and specialized arrays did, but then this single joint dependency is duplicated in two places. Maintenance is more difficult than it should be.

To fix this problem, I added a new word, require-when. The code above, as a top-level form in the locals vocab, would be replaced with

"prettyprint" "locals.prettyprint" require-when

The logic is: if the prettyprint vocab is already loaded, then locals.prettyprint will be loaded. Otherwise, the dependency is registered with the module system, so if prettyprint is loaded later by something else, then locals.prettyprint will be as well.

I'm pretty satisfied with this solution to a somewhat long-standing problem in Factor. I wonder how other module systems solve the same problem. I have this implemented in a branch in my repository and it should be incorporated into mainline Factor soon.