Existing models for I/O in side-effect free languages focus on functional languages, which are usually based on a largely deterministic reduction strategy, allowing for a strict sequentialization of I/O operations. In concurrent logic programming languages a model is used which allows for don't care non-determinism; the sequentialization of I/O is extensional rather than intensional. We apply this model to equational languages, which are closely related to functional languages, but exhibit don't care non-determinism. The semantics are formulated as constrained narrowing, a relation that contains the rewrite relation, and is contained in the narrowing relation. We present constrained narrowing and some of its properties; a constructive method to transform conventional term rewriting systems to constrained narrowing systems; and a discussion on requirements for an implementation.

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Department of Computer Science [CS]
Extensible programming environments

Walters, H.R, & Kamperman, J.F.T. (1995). A model for I/O in equational languages with don't care non-determinism. Department of Computer Science [CS]. CWI.