Procedure properties

You can associate arbitrary properties with any procedure. Each property is a (key, value)-pair. Usually the key is a symbol, but it can be any object.

The preferred way to set a property is using an option-pair in a lambda-expression. For example, to set the setter property of a procedure to my-set-car do the following:

(define my-car
  (lambda (arg) setter: my-set-car (primitive-car arg)))

The system uses certain internal properties: 'name refers to the name used when a procedure is printed; 'emacs-interactive is used to implement Emacs interactive specification; 'setter is used to associate a setter procedure.

Procedure: procedure-property proc key [default] ¶

Get the property value corresponding to the given key. If proc has no property with the given key, return default (which defaults to #f) instead.

Procedure: set-procedure-property! proc key value ¶

Associate the given value with the key property of proc.

To change the print name of the standard + procedure (probably not a good idea!), you could do:

(set-procedure-property! + 'name 'PLUS)

Note this only changes the name property used for printing:

+ ⇒ #<procedure PLUS>
(+ 2 3) ⇒ 5
(PLUS 3 4) ⇒ ERROR

As a matter of style, it is cleaner to use the define-procedure form, as it is a more declarative interface.

Syntax: define-procedure name [propname: propvalue] ... method ... ¶

Defines name as a compound procedure consisting of the specified methods, with the associated properties. Applying name select the "best" method, and applies that. See the following section on generic procedures.

For example, the standard vector-ref procedure specifies one method, as well as the setter property:

(define-procedure vector-ref
  setter: vector-set!
  (lambda (vector::vector k ::int)
    (invoke vector 'get k)))

You can also specify properties in the lambda body:

(define (vector-ref vector::vector k ::int)
    setter: vector-set!
    (invoke vector 'get k))

• Standard properties