LetS: An Expressional Loop Notation

Abstract

Many loops can be more easily understood and manipulated if they are viewed as being built up out of operations on sequences of values. A notation is introduced which makes this viewpoint explicit. Using it, loops can be represented as compositions of functions operating on sequences of values. A library of standard sequence functions is provided along with facilities for defining additional ones. The notation is not intended to be applicable to every kind of loop. Rather, it has been simplified wherever possible so that straightforward loops can be represented extremely easily. The expressional form of the notation makes it possible to construct and modify such loops rapidly and accurately. The implementation of the notation does not actually use sequences but rather compiles loop expressions into iterative loop code. As a result, using the notation leads to no reduction in run time efficiency.