Para-functional programming: a paradigm for programming multiprocessor systems

One of the most important pragmatic advantages of functional languages is that concurrency in a program is <i>implicit</i> -- there is no need for special constructs to express parallelism as is required in most conventional languages. Furthermore, it is fairly easy for compilers to automatically determine the concurrency as a step toward decomposing a program for execution on a suitable parallel architecture. Yet it is often the case that one knows precisely the <i>optimal decomposition</i> for execution on a particular machine, and one can never expect a compiler to determine such optimal mappings in all cases. This paper is concerned with ways to allow the programmer to <i>explicitly</i> express this mapping of program to machine, by using annotations that, given a few minor constraints, cannot alter the functional semantics of the program. We show through several detailed examples the expressiveness and conciseness of the resulting "para-functional" programming methodology, using an experimental language called <i>ParAlfl</i> based on our ideas. We also give a formal denotational description of the mapping semantics using a notion of <i>execution trees.</i>This research was supported in part by NSF Grants DCR-8403304 and DCR-8451415, and a Faculty Development Award from IBM.

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