On the Expressive Power of Concurrent Constraint Programming Languages

The tcc paradigm is a formalism for timed concurrent constraint programming. Several tcc languages differing in their way of expressing infinite behavior have been proposed in the literature. In this paper we study the expressive power of some of these languages. In particular, we show that: (1) recursive procedures with parameters can be encoded into parameterless recursive procedures with dynamic scoping, and viceversa. (2) replication can be encoded into parameterless resursive procedures with static scoping, and viceversa. (3) the languages from (1) are strictly more expressive than the languages from (2). Furthermore, we show that behavioral equivalence is undecidable for the languages from (1), but decidable for the languages from (2). The undecidability result holds even if the process variables take values from a fixed finite domain. ∗The contribution of Mogens Nielsen and Frank D. Valencia to this research has been supported by Basic Research in Computer Science, Centre of the Danish National Research Foundation. †The contribution of Catuscia Palamidessi to this research has been supported by the NSF-POWRE grant EIA-0074909.