On iterative constructs

Abstract

The wheel is repeatedly reinvented because it is a good idea. Perhaps Anson's "A Generalized Iterative Construct and Its Semantics" [1] confirms that “A Generalized Control Structure and Its Formal Definition” [2], and the earlier “An Alternative Control Structure and its Formal Definition” [3] presented good ideas. However, there are several misstatements in [1] that should be corrected. The discussion of these issues is made a bit academic by the four-year delay between Anson's submission of his paper (which apparently coincided with the publication of [2]) and the publication of [1]. In that time a generalization of both schemes has been published as a Technical Report [6] and has been submitted for publication. In this generalization the decision about whether a command is iterating or terminating can be made during execution, and the semantics must be that of DO TERM. Further generalizations make the seman tics of the constructs more practical, since side-effects are accurately treated in all cases. A method for reducing the length of guards and avoiding duplicated subexpressions is also provided.