Nondeterminism with Referential Transparency in Functional Programming Languages

Abstract

Procedural programmers have found nondeterminism to be unavoidable in many parallel programs (e.g. operating systems). If function programming languages are to be usable as general-purpose parallel-programming languages then provision of some nondeterministic language construct appears to be necessary. Existing constructs for nondeterminism are not functional and are not compatible with the mathematical foundations of functional programming, which require that the value of a function be uniquely determined by the values of its arguments. We propose to solve this problem by placing the nondeterminism in pseudo-data. A program is passed an infinite tree of two-valued decisions, along with its input. These decisions may be fixed at run time, thereby permitting nondeterminism. Once fixed, a decision remains unchanged, so equivalent expressions must always have the same value. The approach generalizes so that a program can make use of other run-time information such as the current time or the current amounts of available storage.