8.4 Literal sets and Conventions

Sometimes the same literals are recognized in a number of different places. The most common example is the literals for fully expanded programs, which are used in many analysis and transformation tools. Specifying literals individually is burdensome and error-prone. As a remedy, syntax/parse offers literal sets. A literal set is defined via define-literal-set and used via the #:literal-set option of syntax-parse.

Examples:

> (define-literal-set def-litset       (define-values define-syntaxes))

> (syntax-parse #'(define-syntaxes (x) 12)       #:literal-sets (def-litset)       [(define-values (x:id ...) e:expr) 'v]       [(define-syntaxes (x:id ...) e:expr) 's])

's

The literals in a literal set always refer to the phase-0 bindings of the enclosing module. For example:

Examples:

> (module common racket/base       (define x 'something)       (provide x))

> (module lits racket/base       (require syntax/parse 'common)       (define-literal-set common-lits (x))       (provide common-lits))

In the literal set common-lits, the literal x always recognizes identifiers bound to the variable x defined in module 'common.

When a literal set is used with the #:phase phase-expr option, the literals’ fixed bindings are compared against the binding of the input literal at the specified phase. Continuing the example:

Examples:

> (require syntax/parse 'lits (for-syntax 'common))

> (syntax-parse #'x #:literal-sets ([common-lits #:phase 1])       [x 'yes]       [_ 'no])

'yes

The occurrence of x in the pattern matches any identifier whose binding at phase 1 is the x from module 'common.

Examples:

> (define-conventions xyz-as-ids       [x id] [y id] [z id])

> (syntax-parse #'(a b c 1 2 3)       #:conventions (xyz-as-ids)       [(x ... n ...) (syntax->datum #'(x ...))])

'(a b c)

> (define-conventions xn-prefixes       [#rx"^x" id]       [#rx"^n" nat])

> (syntax-parse #'(a b c 1 2 3)       #:conventions (xn-prefixes)       [(x0 x ... n0 n ...)        (syntax->datum #'(x0 (x ...) n0 (n ...)))])

'(a (b c) 1 (2 3))

Local conventions, introduced with the #:local-conventions keyword argument of syntax-parse and syntax class definitions, may refer to local bindings:

Examples:

> (define-syntax-class (nat> bound)       (pattern n:nat                #:fail-unless (> (syntax-e #'n) bound)                              (format "expected number > ~s" bound)))

> (define-syntax-class (natlist> bound)       #:local-conventions ([N (nat> bound)])       (pattern (N ...)))

> (define (parse-natlist> bound x)       (syntax-parse x         #:local-conventions ([NS (natlist> bound)])         [NS 'ok]))

> (parse-natlist> 0 #'(1 2 3))

'ok

> (parse-natlist> 5 #'(8 6 4 2))

?: expected number > 5 at: 4