Referencing to a class generated in another language/generator

The MPS approach to model generation is very similar to normal compilers. It should be possible to generate only a part of a project. In java you can compile only one source file, in MPS the smallest unit is a model.

If you generate one model, referenced models could be not generated yet. So you cannot rely on their output or intermediate models. Instead you should produce output given the very first existing input models.

There are many ways to achieve that goal in your own language. But the idea is still the same. The number of external nodes you need to generate a model is quite few. You probably don't need every existing model, but only ones you refer to. Besides, you don't need every node from that models except the "exported" or "public" ones. Talking about java, the only data you need about existing classes is their methods, fields and inner classifiers, i.e. an interface, not the implementation. The solution is to import this external APIs into our model in the beginning and process them along with the current model.

Let's consider an example. We have two models X and Y. The first one contains higher-level abstraction of classifier (like state machine). The later one refers to the first one.

Here is the reduction sequence:
X: ConceptA -> ConceptB -> Classifier -> java class
Y: ConceptARef -> ConceptBRef -> ClassifierType -> ref to class in .java file

If ConceptA and ConceptARef live in the same model, everything is clear. When they are separated there is a problem: we cannot refer to ConceptB from ConceptBRef. The target doesn't exist while we generate Y. It only appears in intermediate models of X. OK, we need to make this reference internal.

The obvious ways are:

1. Whenever you need to refer to ConceptB, copy it into the current transient model, as a root, but mark it somehow to delete at the last step (to avoid text generation). That is exactly what Java compiler does when you import external classes: it reads them (without method bodies, of course), stores their interface in the memory and probably processes them.

2. Create an internal reference concept, containing all required information about the target (method call can contain types for all parameters for example). For baseLanguage we created such internal concepts in generator-specific baseLanguageInternal language.