2 Syntactic Categories
Rhombus expansion involves various syntactic categories that determine different kinds of expansion contexts. The specific set of contexts depends on the language, and not the Rhombus expander, but here are some possible contexts:
declarations (in a module’s immediate body or at the top level)
definitions
expressions
bindings (like match patterns, but everywhere)
In Racket’s expander, a few core contexts are reflected by syntax-local-context, but the Racket expander has only one kind of transformer that is represented by one kind of compile-time value: a procedure of arity 1. Nevertheless, some macros work only in, say, definition positions or in a module body. Rhombus expansion instead expects different kinds of compile-time values for different expansion contexts, so a mapping can declare where it’s meant to be used.
The Rhombus expander is parameterized over the way that different kinds of compile-time values for different contexts are recognized, but they are expected to be implemented through structure-type properties. A compile-time value can then implement multiple kinds of transformers to create a mapping that works in multiple contexts. For example, the previous <> operator is useful in both expression and binding contexts, with a suitable meaning in each context.
Different contexts may also consult different mapping spaces in the sense of (provide (for-space ....)). Contexts like declarations, definitions, and expressions are likely to use the default space, while binding, require, and provide contexts might use their own spaces. For example, in the prototype language supplied with this proposal, operator and bind.macro can both bind <> because the former binds in the default space and the latter in the binding space. The Rhombus expander itself is, again, parameterized over the way that mapping spaces are used.
The relevant syntactic category for a shrubbery is determined by its surrounding forms, and not inherent to the shrubbery. For example, Posn(x, y) or x <> y in the example means one thing as an expression and another as a binding. Exactly where the contexts reside in a module depends on a specific Rhombus language that is built on the Rhombus expander. Meanwhile, a full Rhombus language can have different or more syntactic categories than the ones listed above. A Rhombus language likely allows extensions to create even more contexts, just like Racket program can have more contexts through syntactic extensions, such as match or Typed Racket.