1.2.1 Modules and Reusable Syntax Classes

As demonstrated in the Introduction, the simplest place to define a syntax class is within the macro definition that uses it. But this location, of course, limits the scope of the syntax class to the one client macro. Creating reusable syntax classes is slightly complicated, however, by the Racket phase level separation. A syntax class defined within a module cannot be used by macros in the same module; it is defined at the wrong phase.

> (module phase-mismatch-mod racket
    (require syntax/parse (for-syntax syntax/parse))
    (define-syntax-class foo
      (pattern (a b c)))
    (define-syntax (macro stx)
      (syntax-parse stx
        [(_ f:foo) #'(+ f.a f.b f.c)])))

syntax-parse: not defined as syntax class at: foo

In the module above, the syntax class foo is defined at phase level 0. The reference to foo within macro, however, is at phase level 1, being the implementation of a macro transformer. (Needing to require syntax/parse twice, once normally and once for-syntax is another sign of the phase level incompatibility.) The only way to define reusable syntax classes that can be used within macros is to define them in a separate module and require that module for-syntax.

> (module stxclass-mod racket
    (require syntax/parse)
    (define-syntax-class foo
      (pattern (a b c)))
    (provide foo))
> (module macro-mod racket
    (require (for-syntax syntax/parse
    (define-syntax (macro stx)
      (syntax-parse stx
        [(_ f:foo) #'(+ f.a f.b f.c)]))
    (provide macro))
> (require 'macro-mod)
> (macro (1 2 3))


If the syntax classes refer to keywords, or if they compute expressions via syntax templates, then the module containing the syntax classes must generally require the keywords or bindings used in the syntax templates for-template.

> (module arith-keywords-mod racket
    (define-syntax plus (syntax-rules ()))
    (define-syntax times (syntax-rules ()))
    (provide plus times))
> (module arith-stxclass-mod racket
    (require syntax/parse
             (for-template 'arith-keywords-mod
    (define-syntax-class arith
      #:literals (plus times)
      (pattern n:nat
               #:with expr #'n)
      (pattern (plus a:arith b:arith)
               #:with expr #'(+ a.expr b.expr))
      (pattern (times a:arith b:arith)
               #:with expr #'(* a.expr b.expr)))
    (provide arith))
> (module arith-macro-mod racket
    (require (for-syntax syntax/parse
    (define-syntax (arith-macro stx)
      (syntax-parse stx
        [(_ a:arith)
         #'(values 'a.expr a.expr)]))
    (provide arith-macro
             (all-from-out 'arith-keywords-mod)))
> (require 'arith-macro-mod)
> (arith-macro (plus 1 (times 2 3)))

'(+ 1 (* 2 3))


In 'arith-stxclass-mod, the module 'arith-keywords-mod must be required for-template because the keywords are used in phase-0 expressions. Likewise, the module racket must be required for-template because the syntax class contains syntax templates involving + and * (and, in fact, the implicit #%app syntax). All of these identifiers (the keywords plus and times; the procedures + and *; and the implicit syntax #%app) must be bound at “absolute” phase level 0. Since the module 'arith-stxclass-mod is required with a phase level offset of 1 (that is, for-syntax), it must compensate with a phase level offset of -1, or for-template.