Editing Diagrams/Dev/Expression (section)

=== Pros ===

* No more backend type parameter "infecting" the types.  2D diagrams have the simple type <code>Diagram R2</code>.

* Would allow getting rid of the dummy "backend token" parameter to many functions

* The above makes it much easier to hand a diagram to different backends, or even to use multiple backends at once.

* More graceful degrading of service---if a diagram has a single primitive that is not supported by backend X, it is still possible to use that diagram with backend X, for the price of one missing primitive.

* In conjunction with the above, this should make it possible to add a choice combinator, <code>orElse :: Diagram v -> Diagram v -> Diagram v</code>, which instructs a backend to use the first diagram it fully supports.  This would make it even easier to construct diagrams that are portable between different backends, letting the user explicitly supply fallbacks in cases of differing features.

* As diagrams continues to grow it is becoming increasingly clear that we will need some testing frameworks. If a diagram is defined as <code>Diagram R2</code> we would be able to create tests that pass if they produce a diagram that matches a reference diagram.

* Consistency with attributes. We already use dynamic typing for attributes and we may want to have the backends handle attributes and primitives they do not support the same way.