Building a Backtracking Facility in Smalltalk Without Kernel Support

Abstract

Languages like Snobol, Prolog, and Icon were designed with backtracking facilities from the outset and these facilities are deeply intertwined with the implementation. Retrofitting a backtracking facility in a language that wasn't designed for it has never been achieved. We report on an experiment to retrofit Smalltalk with a backtracking facility. The facility is provided through a small number of primitives written in the language (no modifications to the kernel were made). The ability to do this is a direct result of the power provided by the objectification of contexts.