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define/ with-syntax
syntax-local-value/ catch
Version: 5.0

13 Syntax

Ryan Culpepper <ryanc@racket-lang.org>

 (require unstable/syntax)

This library is unstable; compatibility will not be maintained. See Unstable for more information.

The current contextual syntax object, defaulting to #f. It determines the special form name that prefixes syntax errors created by wrong-syntax.

(wrong-syntax stx format-string v ...)  any
  stx : syntax?
  format-string : string?
  v : any/c
Raises a syntax error using the result of (current-syntax-context) as the “major” syntax object and the provided stx as the specific syntax object. (The latter, stx, is usually the one highlighted by DrRacket.) The error message is constructed using the format string and arguments, and it is prefixed with the special form name as described under current-syntax-context.


  > (wrong-syntax #'here "expected ~s" 'there)

  ?: expected there

  > (parameterize ((current-syntax-context #'(look over here)))
      (wrong-syntax #'here "expected ~s" 'there))

  eval:4:0: look: expected there at: here in: (look over here)

A macro using wrong-syntax might set the syntax context at the very beginning of its transformation as follows:
  (define-syntax (my-macro stx)
    (parameterize ((current-syntax-context stx))
      (syntax-case stx ()
Then any calls to wrong-syntax during the macro’s transformation will refer to my-macro (more precisely, the name that referred to my-macro where the macro was used, which may be different due to renaming, prefixing, etc).

(define/with-syntax pattern expr)
Definition form of with-syntax. That is, it matches the syntax object result of expr against pattern and creates pattern variable definitions for the pattern variables of pattern.


  > (define/with-syntax (px ...) #'(a b c))
  > (define/with-syntax (tmp ...) (generate-temporaries #'(px ...)))
  > #'([tmp px] ...)

  #<syntax:7:0 ((a5 a) (b6 b) (c7 c))>

Evaluates expr and binds it to id as a pattern variable, so id can be used in subsequent syntax patterns.


  > (define-pattern-variable name #'Alice)
  > #'(hello name)

  #<syntax:9:0 (hello Alice)>

(with-temporaries (temp-id ...) . body)
Evaluates body with each temp-id bound as a pattern variable to a freshly generated identifier.


  > (with-temporaries (x) #'(lambda (x) x))

  #<syntax:10:0 (lambda (x8) x8)>

(generate-temporary [name-base])  identifier?
  name-base : any/c = 'g
Generates one fresh identifier. Singular form of generate-temporaries. If name-base is supplied, it is used as the basis for the identifier’s name.

Generates a list of n fresh identifiers.

Parameter for tracking disappeared uses. Tracking is “enabled” when the parameter has a non-false value. This is done automatically by forms like with-disappeared-uses.

(with-disappeared-uses stx-expr)
  stx-expr : syntax?
Evaluates the stx-expr, catching identifiers looked up using syntax-local-value/catch. Adds the caught identifiers to the 'disappeared-uses syntax property of the resulting syntax object.

Evaluates the body-expr, catching identifiers looked up using syntax-local-value/catch. Returns two values: the result of body-expr and the list of caught identifiers.

(syntax-local-value/catch id predicate)  any/c
  id : identifier?
  predicate : (-> any/c boolean?)
Looks up id in the syntactic environment (as syntax-local-value). If the lookup succeeds and returns a value satisfying the predicate, the value is returned and id is recorded (“caught”) as a disappeared use. If the lookup fails or if the value does not satisfy the predicate, #f is returned and the identifier is not recorded as a disappeared use.

If not used within the extent of a with-disappeared-uses form or similar, has no effect.

(record-disappeared-uses ids)  void?
  ids : (listof identifier?)
Add ids to the current disappeared uses.

If not used within the extent of a with-disappeared-uses form or similar, has no effect.

(format-symbol fmt v ...)  symbol?
  fmt : string?
  v : (or/c string? symbol? identifier? keyword? char? number?)
Like format, but produces a symbol. The format string must use only ~a placeholders. Identifiers in the argument list are automatically converted to symbols.


  > (format-symbol "make-~a" 'triple)


(format-id lctx    
  [#:source src    
  #:props props    
  #:cert cert]    
  v ...)  identifier?
  lctx : (or/c syntax? #f)
  src : (or/c syntax? #f) = #f
  props : (or/c syntax? #f) = #f
  cert : (or/c syntax? #f) = #f
  fmt : string?
  v : (or/c string? symbol? identifier? keyword? char? number?)
Like format-symbol, but converts the symbol into an identifier using lctx for the lexical context, src for the source location, props for the properties, and cert for the inactive certificates. (See datum->syntax.)

The format string must use only ~a placeholders. Identifiers in the argument list are automatically converted to symbols.


  > (define-syntax (make-pred stx)
      (syntax-case stx ()
        [(make-pred name)
         (format-id #'name "~a?" (syntax-e #'name))]))
  > (make-pred pair)


  > (make-pred none-such)

  reference to undefined identifier: none-such?

  > (define-syntax (better-make-pred stx)
      (syntax-case stx ()
        [(better-make-pred name)
         (format-id #'name #:source #'name
                    "~a?" (syntax-e #'name))]))
  > (better-make-pred none-such)

  reference to undefined identifier: none-such?

(Scribble doesn’t show it, but the DrRacket pinpoints the location of the second error but not of the first.)

(internal-definition-context-apply intdef-ctx    
  stx)  syntax?
  intdef-ctx : internal-definition-context?
  stx : syntax?
Applies the renamings of intdef-ctx to stx.

(syntax-local-eval stx [intdef-ctx])  any
  stx : syntax?
  intdef-ctx : (or/c internal-definition-context? #f) = #f
Evaluates stx as an expression in the current transformer environment (that is, at phase level 1), optionally extended with intdef-ctx.


  > (define-syntax (show-me stx)
      (syntax-case stx ()
        [(show-me expr)
           (printf "at compile time produces ~s\n"
                   (syntax-local-eval #'expr))
           #'(printf "at run time produes ~s\n"
  > (show-me (+ 2 5))

  at compile time produces 7

  at run time produes 7

  > (define-for-syntax fruit 'apple)
  > (define fruit 'pear)
  > (show-me fruit)

  at compile time produces apple

  at run time produes pear

The subsequent bindings were added by Sam Tobin-Hochstadt.

(with-syntax* ([pattern stx-expr] ...)
  body ...+)
Similar to with-syntax, but the pattern variables are bound in the remaining stx-exprs as well as the bodys, and the patterns need not bind distinct pattern variables; later bindings shadow earlier bindings.


  > (with-syntax* ([(x y) (list #'val1 #'val2)]
                   [nest #'((x) (y))])

  #<syntax:22:0 ((val1) (val2))>

(syntax-map f stxl ...)  (listof A)
  f : (-> syntax? A)
  stxl : syntax?
Performs (map f (syntax->list stxl) ...).


  > (syntax-map syntax-e #'(a b c))

  '(a b c)