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Design pattern: Memento

Description: "Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later."

This pattern can be used with both classes and prototypes.

In this sample implementation, mementos are represented using dynamic objects holding the saved state. An alternative would be to create mementos as parametric object proxies with a corresponding parametric object to trivially access the sate. The protocol for using mementos would be the same but without using dynamic objects anf object dynamic predicates. The best solution would depend notably on the need of mementos to survive backtracking.

The originator state is here simply represented as an opaque term. It could be represented in a practical case by e.g. a list of pairs PredicateIndicator-ListOfClauses for all the originator dynamic predicates. Restoring a memento would then become walking the list and, for each pair, retracting all clauses for the predicate and then asserting all listed clauses. These operations of saving and restoring state from mementos are inherently expensive but could be minimized by saving and restoring only the partial state affected by a given operation performed on the originator.

Print Logtalk, Prolog backend, and kernel versions (if running as a notebook):

%versions

Start by loading the design pattern sample implementation:

logtalk_load(design_patterns('behavioral/memento/loader')).

Sample sequence of operations from user, which plays the role of the caretaker in this design pattern:

originator::(
                set(s1),
                set(s2),
                save_to_memento(Momento1),
                set(s3),
                save_to_memento(Momento2),
                set(s4),
                restore_from_memento(Momento1),
                set(s5),
                restore_from_memento(Momento2)
        ).

<!-- Originator: Setting state to s1 Originator: Setting state to s2 Originator: Saving to Memento. Originator: Setting state to s3 Originator: Saving to Memento. Originator: Setting state to s4 Originator: State after restoring from Memento: s2 Originator: Setting state to s5 Originator: State after restoring from Memento: s3 Momento1 = o1, Momento2 = o2

true. -->