The -calculus with synchronous output and mixed-guarded choices is strictly more expressive than the -calculus with asynchronous output and no choice. As a corollary, Palamidessi recently proved that there is no fully compositional encoding from the former into the latter that preserves divergence-freedom and symmetries. This paper shows

this paper, one aspect of the construction of distributed programs is ad- This research was supported in part by National Science Foundation Grant MCS 76-22360

Recent attempts at incorporating concurrency into functional languages have identified synchronous communication via shared channels as a promising primitive. An additional useful feature found in many proposals is a nondeterministic choice operator. Similar in nature to the CSP alternative command, this operator allows different possible actions to be guarded by sends or receives. Choice is difficult to implement in a distributed environment because it requires offering many potential communications but closing only one. In this paper we present the first distributed, deadlock-free algorithm for choice. Keywords: Distributed protocols, channels, synchronous communication, choice operator, CSP alternative command. 1. Introduction In 1978, C.A.R. Hoare introduced a now-classic paradigm for parallel programming called communicating sequential processes, or CSP [12]. CSP presented a new approach to concurrent programming, and its legacy has continued to influence wellfounded pr...