Compiler redesign

[timothee-haudebourg]

This PR aims at redesigning the compiler model to overcome its current limitations.

Current limitations

• Declaring multiple values on a functional properties will raise an error, even if the values are logically the same;
• Because of the above, it is impossible to write a unification stage that merges equivalent (blank) nodes, leading to a lot of spurious errors with union/intersection types/layouts (False type mismatch error using union types #73).
• Some declarations are dependent on other declarations that must have been given to the compiler before, otherwise it will raise an error. For instance, the class of a resource must be declared before a property of this class can be declared on the resource.
• Different facets of the same resource (for instance if the resource is both a type and a layout) are completely separated in the final compiled model. This means that properties shared between the traits are copied. This also adds some annoying queries to the model during code generation.
• Because of the above, keeping track of the class hierarchy is also nearly impossible.
• The model is weirdly different from OWL, which makes conversion from/to RDF slightly inaccurate.
• Because of the above, it is (again) impossible to write a unification stage that merges equivalent (blank) nodes, leading to a lot of spurious errors with union/intersection types/layouts.

Goal

• Relax the constraints on the intermediate model (e.g. declaring multiple values on a functional property will not immediately raise an error).
• Get closer to OWL. This will allow us to very simply write a conversion from/to RDF. This is essential to define the unification stage.
• Do not separate traits in the final compiled model. This actually simplifies the compilation a lot and give more freedom to the code generators. This also lays the ground for a full implementation of class hierarchy.

Related issues

This should fix or greatly simplify the following issues:

• False type mismatch error using union types #73 (will fix)
• Type hierarchy support #32
• Type check layouts #31
• Better support for XSD datatypes #29

Todo

• [core] Merge RDF traits in the Node type.
• [core] Get closer to OWL.
• [build] Relax all the constraints.
• [build] Remove now unnecessary allocation stage.
• [build] Move the intersection algo away from the parser.
• [build] Define an iterator over the properties of a node, and define the RDF export feature from it.
• [build] Define the (early) unification stage using the RDF export feature.
• [syntax] Update the parser.
• [modules] Update the code generators.
• Test.