1.9 Unwindable State

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1.9Unwindable State🔗 i

procedure
( syntax-parse-state-ref key[default])→any/c
key:any/c
default:default/c=(lambda ()(error ....))

procedure
( syntax-parse-state-set! keyvalue)→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! keyvalue[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:expranswer:expr... ]))

> (syntax-parse #'(cond [A=> B][else C])
[(_ c:cond-clause... )(syntax-parse-state-ref 'literalsnull )])

'(#<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 cons ed 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:expranswer: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.

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