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It's often useful to associate a piece of additional information with a Scheme object even though that object does not have a dedicated slot available in which the additional information could be stored. Object properties allow you to do just that.
Guile's representation of an object property is a procedure-with-setter
(see Procedures with Setters) that can be used with the generalized
form of set! (REFFIXME) to set and retrieve that property for any
Scheme object. So, setting a property looks like this:
(set! (my-property obj1) value-for-obj1)
(set! (my-property obj2) value-for-obj2)
And retrieving values of the same property looks like this:
(my-property obj1)
=>
value-for-obj1
(my-property obj2)
=>
value-for-obj2
To create an object property in the first place, use the
make-object-property procedure:
(define my-property (make-object-property))
Create and return an object property. An object property is a procedure-with-setter that can be called in two ways.
(set! (property obj)val)sets obj's property to val.(property obj)returns the current setting of obj's property.
A single object property created by make-object-property can
associate distinct property values with all Scheme values that are
distinguishable by eq? (including, for example, integers).
Internally, object properties are implemented using a weak key hash table. This means that, as long as a Scheme value with property values is protected from garbage collection, its property values are also protected. When the Scheme value is collected, its entry in the property table is removed and so the (ex-) property values are no longer protected by the table.