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1.7 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.


(define-literal-set id maybe-phase maybe-imports maybe-datum-literals
  (literal ...))
literal = literal-id
  | (pattern-id literal-id)
maybe-phase = 
  | #:for-template
  | #:for-syntax
  | #:for-label
  | #:phase phase-level
maybe-datum-literals = 
  | #:datum-literals (datum-literal ...)
maybe-imports = 
  | #:literal-sets (imported-litset-id ...)
Defines id as a literal set. Each literal can have a separate pattern-id and literal-id. The pattern-id determines what identifiers in the pattern are treated as literals. The literal-id determines what identifiers the literal matches. If the #:literal-sets option is present, the contents of the given imported-litset-ids are included.

> (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])


The literals in a literal set always refer to the bindings at phase phase-level relative to the enclosing module. If the #:for-template option is given, phase-level is -1; #:for-syntax means 1, and #:for-label means #f. If no phase keyword option is given, then phase-level is 0.

For example:

> (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.

The following module defines an equivalent literal set, but imports the 'common module for-template instead:

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

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:

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


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


(literal-set->predicate litset-id)

Given the name of a literal set, produces a predicate that recognizes identifiers in the literal set. The predicate takes one required argument, an identifier id, and one optional argument, the phase phase at which to examine the binding of id; the phase argument defaults to (syntax-local-phase-level).

> (define kernel? (literal-set->predicate kernel-literals))
> (kernel? #'lambda)


> (kernel? #'#%plain-lambda)


> (kernel? #'define-values)


> (kernel? #'define-values 4)



(define-conventions name-id convention-rule ...)

convention-rule = (name-pattern syntax-class)
name-pattern = exact-id
  | name-rx
syntax-class = syntax-class-id
  | (syntax-class-id expr ...)
Defines conventions that supply default syntax classes for pattern variables. A pattern variable that has no explicit syntax class is checked against each name-pattern, and the first one that matches determines the syntax class for the pattern. If no name-pattern matches, then the pattern variable has no syntax class.

> (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:

> (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))


> (parse-natlist> 5 #'(8 6 4 2))

?: expected number > 5

  at: 4

  in: (8 6 4 2)

  parsing context:

   while parsing nat>

    term: 4

    location: eval:21.0

   while parsing natlist>

    term: (8 6 4 2)

    location: eval:21.0