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syntax-parse-state-ref
syntax-parse-state-set!
syntax-parse-state-update!
syntax-parse-state-cons!
syntax-parse-track-literals

1.9 Unwindable State🔗

procedure

(syntax-parse-state-ref key [default])  any/c

  key : any/c
  default : default/c = (lambda () (error ....))

procedure

(syntax-parse-state-set! key value)  void?

  key : any/c
  value : any/c

procedure

(syntax-parse-state-update! key    
  update    
  [default])  void?
  key : any/c
  update : (-> any/c any/c)
  default : default/c = (lambda () (error ....))

procedure

(syntax-parse-state-cons! key value [default])  void?

  key : any/c
  value : any/c
  default : default/c = null
Get or update the current syntax-parse state. Updates to the state are unwound when syntax-parse backtracks. Keys are compared using eq?.

The state can be updated only within ~do patterns (or #:do blocks). In addition, syntax-parse automatically adds identifiers that match literals (from ~literal patterns and literals declared with #:literals, but not from ~datum or #:datum-literals) under the key 'literals.

Examples:
> (define-syntax-class cond-clause
    #:literals (=> else)
    (pattern [test:expr => ~! answer:expr ...])
    (pattern [else answer:expr ...])
    (pattern [test:expr answer:expr ...]))
> (syntax-parse #'(cond [A => B] [else C])
    [(_ c:cond-clause ...) (syntax-parse-state-ref 'literals null)])

'(#<syntax:eval:2:0 else> #<syntax:eval:2:0 =>>)

Added in version 6.11.0.4 of package base.

procedure

(syntax-parse-track-literals stx    
  [#:introduce? introduce?])  syntax?
  stx : syntax?
  introduce? : any/c = #t
Add a 'disappeared-use syntax property to stx containing the information stored in the current syntax-parse state under the key 'literals. If stx already has a 'disappeared-use property, the added information is consed onto the property’s current value.

Due to the way syntax-parse automatically adds identifiers that match literals to the state under the key 'literals, as described in the documentation for syntax-parse-state-ref, syntax-parse-track-literals can be used to automatically add any identifiers used as literals to the 'disappeared-use property.

If syntax-parse-track-literals is called within the dynamic extent of a syntax transformer (see syntax-transforming?), introduce? is not #f, and the value in the current syntax-parse state under the key 'literals is a list, then syntax-local-introduce is applied to any identifiers in the list before they are added to stx’s 'disappeared-use property.

Most of the time, it is unnecessary to call this function directly. Instead, the #:track-literals option should be provided to syntax-parse, which will automatically call syntax-parse-track-literals on syntax-valued results.

Examples:
> (define-syntax-class cond-clause
    #:literals (=> else)
    (pattern [test:expr => ~! answer:expr ...])
    (pattern [else answer:expr ...])
    (pattern [test:expr answer:expr ...]))
> (syntax-property
   (syntax-parse #'(cond [A => B] [else C])
     [(_ c:cond-clause ...) (syntax-parse-track-literals #'#f)])
   'disappeared-use)

'(#<syntax:eval:4:0 else> #<syntax:eval:4:0 =>>)

Added in version 6.90.0.29 of package base.