by **Michael Hanus, Frank Steiner**

*Principles of Declarative Programming (Proc. Joint International Symposium PLILP/ALP'98), *Springer LNCS 1490, pp. 374-390, 1998

© Springer-Verlag

Logic languages can deal with non-deterministic computations via built-in search facilities. However, standard search methods like global backtracking are often not sufficient and a source of many programming errors. Therefore, we propose the addition of a single primitive to logic-oriented languages to control non-deterministic computation steps. Based on this primitive, a number of different search strategies can be easily implemented. These search operators can be applied if the standard search facilities are not successful or to encapsulate search. The latter is important if logic programs interact with the (non-backtrackable) outside world. We define the search control primitive based on an abstract notion of computation steps so that it can be integrated into various logic-oriented languages, but to provide concrete examples we also present the integration of such a control primitive into the multi-paradigm declarative language Curry. The lazy evaluation strategy of Curry simplifies the implementation of search strategies, which also shows the advantages of integrating functions into logic languages.

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