Embedding continuations in procedural objects

Reviewer: Ronald Thomas House

The idea of continuations is old and basically amounts to marking a point during program execution and returning to that state later in execution. The authors discuss first-class continuations, which can be treated like any other data object and freely passed to any point in the program. This facility requires heap-based storage. Using Scheme (a Lisp dialect with such continuations), they show how procedures can be created that implement regulated continuation-based control structures, of which the most ambitious is wind-unwind, which provides automatic execution of cleanup code (a postlude) on exit from an inner scope and similar execution of setup logic (a prelude) on reentry. This is a valuable contribution, since the unrestrained first-class continuation is undoubtedly beyond mastery by the average programmer. The logic presented for all the ideas discussed could be used by a Scheme programmer, but the main value of the paper is probably for language designers, who may be considering these structured features for inclusion in a language, and implementers, who should find the code to be an invaluable source of information. This is a comprehensive and useful paper.