9.2.2 Persistent Object Storage

In order to facilitate the configuration and exchange of data (e.g., optimization results, populations from evolutionary algorithm optimizers) between different invocations of SIESTA, we implemented a simple persistent object storage. Using two functions for writing objects to and reading objects from a file, all of SIESTA's internal data-structures can be stored and retrieved in a text based format. A human readable format was chosen in order to enable inspection and changes with ordinary text editors.

The question how to define equality for any two given objects is closely related to storing and copying objects. Of course, the equality relation has to be reflexive, symmetric, and transitive. Furthermore, the equality relation should not be affected by storing and retrieving objects. Since objects usually contain references to other objects, there is no general copy operator which does the right thing in every application (deep copy vs. shallow copy). Thus the copy operator and the question whether a copied object and the original object should be considered equal depend on the problem at hand.

In SIESTA objects with id's, named objects, and appropriate equality and copying operators are defined and can be extended in an object oriented way. In addition to Common Lisp's four equality operators, we provide the equality operator samep and specialized copy operators. These operators work with the persistent object storage and SIESTA's data-structures as expected.