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 define-struct. Subtypes inherit the property values of their parent types, and subtypes can override an inherited property value with a new value.
|name : symbol?|
|guard : (or/c procedure? #f) = #f|
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-proc 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.
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))|
|> (p? struct:a)|
|> (p? 13)|
|> (define an-a (make-a 'x 'y))|
|> (p? an-a)|
|> (p-ref an-a)|
|> (p? struct:b)|
|> (q-ref struct:c)|
|> (p-ref struct:c)|