Conditional Narrowing without Conditions

by Sergio Antoy, Bernd Bra├čel, Michael Hanus

Proc. of the 5th International ACM SIGPLAN Conference on Principles and Practice of Declarative Programming (PPDP'03), ACM Press, pp. 20-31, 2003
© ACM Press

We present a new evaluation strategy for functional logic programs described by weakly orthogonal conditional term rewriting systems. Our notion of weakly orthogonal conditional rewrite system extends a notion of Bergstra and Klop and covers a large part of programs defined by conditional equations. Our strategy combines the flexibility of logic programming (computation of solutions for logic variables) with efficient evaluation methods from functional programming. In particular, it is the first known narrowing strategy for this class of programs that evaluates ground terms deterministically. This is achieved by a transformation of conditional term rewriting systems (CTRS) into unconditional ones which is sound and complete w.r.t. the semantics of the original CTRS. We show that the transformation preserves weak orthogonality for the terms of interest. This property allows us to apply a relatively efficient evaluation strategy for weakly orthogonal unconditional term rewriting systems (parallel narrowing) on the transformed programs.

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