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.

• a structure type property descriptor, for use with make-struct-type and struct;

• a property predicate procedure, which takes an arbitrary value and returns #t if the value is a descriptor or instance of a structure type that has a value for the property, #f otherwise;

• an property accessor procedure, which returns the value associated with the structure type given its descriptor or one of its instances; if the structure type does not have a value for the property, or if any other kind of value is provided, the exn:fail:contract exception is raised.

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).

Examples:

> (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)

#t

> (p? 13)

#f

> (define an-a (make-a 'x 'y))

> (p? an-a)

#t

> (p-ref an-a)

8

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

> (p? struct:b)

#f

> (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)

9

> (p-ref struct:c)

3