Structured Lucid ∗

Structured Lucid is an ISWIM-like language formed by combining Basic Lucid and USWIM. Structured Lucid is almost a member of the USWIM family, except that a distinction is made between two kinds of function variables: those which denote functions mapping streams (histories) of data objects to streams of data objects, and those which denote streams of functions mapping data objects to data objects. The inference and manipulation rules of Structured Lucid combine Basic Lucid’s facility for implicit reasoning about time with the modular or nested reasoning of USWIM. The distinction between stream functions and streams of functions is inherent in the ideas of Basic Lucid and has a natural operational interpretation. On one hand, a module which computes a stream of functions can be thought of as defining a subcomputation which is carried out while the main computation ‘waits’ or is ‘frozen’; on the other hand, some modules which compute stream functions can be thought of as defining separate computations carried out in parallel with the main computation (a form of coroutine). In general, though, a module may freeze some but not all of its parameters or globals, and for these various combinations of the two interpretations are appropriate.