In Racket 7, the syntax form supports two new template subforms:

• ~@ (“splice”) splices the result of its subtemplate (which must produce a syntax list) into the enclosing list template, and

• ~? (“try”) chooses between alternative subtemplates depending on whether their pattern variables have “absent” values.

These features originated in syntax/parse/experimental/template: the template form was like an extended version of syntax, and ?@ and ?? were the splicing forms and the try/else template forms, respectively. The names were changed to ~@ and ~? to avoid name collisions with other libraries. In Racket 7, the old syntax/parse/experimental/template library just exports the new standard forms under the old names (that is, it exports syntax under the name template, and so on).

In my last post, I talked about macros and referential auxiliary identifiers—what we usually call a macro’s “literals.” Scheme macro systems only get it half right, though, because while they compare identifiers using referential equality (i.e., using the free-identifier=? predicate), they allow literals to refer to nonexistent bindings. While the comparison is well-defined via the definition of free-identifier=?, at a higher level the idea is nonsensical.

In contrast, syntax-parse requires that every literal refer to some binding. (I’ll sometimes refer to this requirement as the is-bound property for short.) This requirement is problematic in a different way. Specifically, this property cannot be checked statically (that is, when the syntax-parse expression containing the literal is compiled).

Macros often have associated auxiliary identifiers (sometimes called keywords or reserved words, although both terms are problematic in Racket). For example, cond has else; class has public, private, etc; unit has import and export.

The fundamental question is what constitutes a use of an auxiliary identifier, and there are two reasonable answers: symbolic equality and referential equality. By symbolic equality I mean, for example, that any identifier written with exactly the letters else is accepted as an else auxiliary form. By referential equality I mean any identifier that refers to (using the standard notions of binding, environments, etc) the binding identified as the else binding.

Danny Yoo had an interesting question on the plt-scheme mailing list recently. At first it seemed like your standard non-hygienic, “I want this to mean something in here” macro question. I used to group macros into three levels of “hygienicness”: the hygienic ones, the ones that are morally hygienic in that the names they introduce are based on their input, and the totally non-hygienic ones that have a fixed name that they stick into the program. An example of the middle set is define-struct, and an example of the third set is a loop construct that binds the name yield in its body.

The third class used to offend me from a purist’s (semi-purist?) perspective. But it’s a very reasonable thing to want to do. Consider the class macro and the names it uses to do interesting things: super, public, field, init, and so on. It depends on those particular names.