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4.3 Structure Type Properties

A structure type property allows per-type information to be associated with a structure type (as opposed to per-instance information associated with a structure value). A property value is associated with a structure type through the make-struct-type procedure (see Creating Structure Types) or through the #:property option of struct. Subtypes inherit the property values of their parent types, and subtypes can override an inherited property value with a new value.

(make-struct-type-property name    
  name : symbol?
  guard : (or/c procedure? #f 'can-impersonate) = #f
  supers : 
(listof (cons/c struct-type-property?
                (any/c . -> . any/c)))
 = null
Creates a new structure type property and returns three values:

If the optional guard is supplied as a procedure, it is called by make-struct-type before attaching the property to a new structure type. The guard must accept two arguments: a value for the property supplied to make-struct-type, and a list containing information about the new structure type. The list contains the values that struct-type-info would return for the new structure type if it skipped the immediate current-inspector control check (but not the check for exposing an ancestor structure type, if any; see Structure Inspectors).

The result of calling guard is associated with the property in the target structure type, instead of the value supplied to make-struct-type. To reject a property association (e.g., because the value supplied to make-struct-type is inappropriate for the property), the guard can raise an exception. Such an exception prevents make-struct-type from returning a structure type descriptor.

If guard is 'can-impersonate, then the property’s accessor can be redirected through impersonate-struct. Otherwise, redirection of the property value through an impersonator is disallowed, since redirection is tantamount to mutation.

The optional supers argument is a list of properties that are automatically associated with some structure type when the newly created property is associated to the structure type. Each property in supers is paired with a procedure that receives the value supplied for the new property (after it is processed by guard) and returns a value for the associated property (which is then sent to that property’s guard, of any).


> (define-values (prop:p p? p-ref) (make-struct-type-property 'p))
> (define-values (struct:a make-a a? a-ref a-set!)
    (make-struct-type 'a #f 2 1 'uninitialized
                      (list (cons prop:p 8))))
> (p? struct:a)


> (p? 13)


> (define an-a (make-a 'x 'y))
> (p? an-a)


> (p-ref an-a)


> (define-values (struct:b make-b b? b-ref b-set!)
    (make-struct-type 'b #f 0 0 #f))
> (p? struct:b)


> (define-values (prop:q q? q-ref) (make-struct-type-property
                                    'q (lambda (v si) (add1 v))
                                    (list (cons prop:p sqrt))))
> (define-values (struct:c make-c c? c-ref c-set!)
    (make-struct-type 'c #f 0 0 'uninit
                      (list (cons prop:q 8))))
> (q-ref struct:c)


> (p-ref struct:c)


Returns #t if v is a structure type property descriptor value, #f otherwise.

Returns #t if v is an accessor procedure produced by make-struct-type-property, #f otherwise.