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?
Creates a new structure type property and returns three values:
an property accessor procedure, which returns the value associated with 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.
> (p? struct:a)
> (p? 13)
> (define an-a (make-a 'x 'y))
> (p? an-a)
> (p-ref an-a)
> (p? struct:b)
v : any/c
Returns #t if v is a structure type property descriptor value, #f otherwise.