Transformation of Left Terminating Programs: the Reordering Problem

Abst ract. An Unfold/Fold transformation system is a source-to-source rewrit-ing methodology devised to improve the efficiency of a program. Any such transformation should preserve the main properties of the initial program: among them, termination. When dealing with logic programs uch as PRO-LOG programs, one is particularly interested in preserving left termination i.e. termination wrt the leftmost selection rule, which is by far the most widely employed of the search rules. Unfortunately, the most popular Unfold/Fold transformation systems ([TS84, Sek91]) do not preserve the above termina-tion property. In this paper we study the reasons why left termination may be spoiled by the application of a transformation operation and we present a transformation system based on the operations of Unfold, Fold and Switch which- if applied to a left terminating programs- yields a program which is left terminating as well. 1 Introduct ion As shown by a number of applications, program transformation is a valuable method