Abstract. We present a lightweight, automated tool for specificationbased testing of declarative programs written in the functional logic programming language Curry and emphasize the usefulness of logic features in its implementation and use. Free variables, nondeterminism and encapsulated search turn out to be elegant and powerful means to express test-data generation.

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Encapsulated search is a key feature of (functional) logic languages. It allows the programmer to access and process different results of a non-deterministic computation within a program. Unfortunately, due to advanced operational features (lazy evaluation, partial values, infinite structures), there is no straightforward definition of the semantics of encapsulated search in functional logic languages. As a consequence, various proposals and implementations are available but a rigorous definition covering all semantical aspects does not exist. In this paper, we analyze the requirements of encapsulated search in a functional logic language like Curry and provide a comprehensive definition that covers weak encapsulation, a modular form of encapsulation, as well as nested applications of search operators. We set up a denotational semantics that distinguishes non-termination and different levels of failures in a computation. The semantics is also the basis of a practical implementation of search operators in the functional logic language Curry.

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Type-based reasoning is popular in functional programming. In particular, parametric polymorphism constrains functions in such a way that statements about their behavior can be derived without consulting function definitions. Is the same possible in a strongly, and polymorphically, typed functional logic language? This is the question we study in this paper. Logical features like nondeterminism and free variables cause interesting effects, which we examine based on examples and address by identifying appropriate conditions that guarantee standard free theorems or inequational versions thereof to hold. We see this case study as a stepping stone for a general theory, not provided here, involving the definition of a logical relation and other machinery required for parametricity arguments appropriate to functional logic languages.