Lazy Database Access with Persistent Predicates ⋆

Programmers need mechanisms to store application specific data that persists multiple program runs. To accomplish this task, they usually have to deal with storage specific code to access files or relational databases. Functional logic programming provides a natural framework to transparent persistent storage through persistent predicates, i.e., predicates with externally stored facts.We extend previous work on persistent predicates for Curry by lazy database access. Results of a database query are only read as much as they are demanded by the application program. We also present a type-oriented approach to convert between database and Curry values which is used to implement lazy access to persistent predicates based on a low level lazy database interface.