4.3 Structure Type Properties

Generic Interfaces provide a high-level API on top of 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;

• a 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 unless a second argument, failure-result, is supplied to the procedure. In that case, if failure-result is a procedure, it is called (through a tail call) with no arguments to produce the result of the property accessor procedure; otherwise, failure-result is itself returned as the result.

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. This option is identical to supplying #t as the can-impersonate? argument and is provided for backwards compatibility.

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).

The optional can-impersonate? argument determines if the structure type property can be redirected through impersonate-struct. If the argument is #f, then redirection is not allowed. Otherwise, the property accessor may be redirected by a struct impersonator.