Editing Diagrams/Dev/Expression (section)

== Static typing ==

This is the approach currently taken.  Each backend has a "token type" and must implement instances of the form <code>Renderable Prim Backend</code>.  Prims get wrapped up in an existential wrapper along with an appropriate <code>Renderable</code> instance.  When a backend goes to render some primitives, it just unwraps them and uses the enclosed <code>Renderable</code> instance; it doesn't even need to know what type they are.

This does mean, however, that the backend type ''must'' show up as a parameter to the <code>Diagram</code> type---otherwise the existentially quanitifed <code>Renderable</code> instances would be useless, as there would be no way to know which backend they are for!  However, this also makes sense, in a way.  If a diagram has type <code>Diagram Foo R2</code>, then we know statically that it can only contain primitives which can be rendered by backend Foo---because any primitive of type <code>P</code> must be wrapped up with a <code>Renderable P Foo</code> instance, and the only way to obtain such an instance is if it is provided by the backend.

More generally, diagrams can be given polymorphic types like

<code>(Backend b R2, Renderable (Path R2) b, Renderable Image b) => Diagram b R2</code>

with one constraint for each different type of primitive contained in the diagram.

=== Pros ===

* Rendering will never fail at runtime.  We know statically that if it typechecks to give a certain diagram to a certain backend, the backend knows what to do with all the primitives it contains.

* Behavior is more transparent to users: if it typechecks, it will work as expected.

=== Cons ===

* The backend type parameter ends up "infecting" quite a lot of types, including UpAnnots, DownAnnots, QDiagram, QDiaLeaf, Subdiagram, SubMap, Prim, DNode, DTree, RNode, RTree.

* If users want to give type signatures to their diagrams, they have to either write some complicated polymorphic type with lots of constraints, or tie their diagram to a specific backend (or use the <code>B</code> hack, which sort of works, but is not perfect---e.g. it makes it hard to use multiple backends at once).