Sacrificing simplicity for convenience: Where do you draw the line?

Abstract

The designers of (functional) programming languages are faced with two occasionally conflicting goals: programmer convenience and semantic simplicity. For example, it is convenient to treat I/O operations as primitive "functions" with side effects, but doing so destroys referential transparency. FL is a functional language that is designed to trade some of the semantic simplicity of a pure language for some of the convenience of a procedural language, by treating I/O operations as primitives with "side effects", but by using a structuring technique that localizes the scope of those effects. In this way, surprisingly little of the semantic simplicity is lost, as can be seen by comparing the underlying algebraic laws of FL with those of its pure counterpart, FP. This paper describes that comparison and shows that, in fact, for programs involving I/O, the structures of the algebraic laws of the two languages are identical! It concludes by showing that this technique cannot be extended to allow assignment statements without incurring a massive loss in the expressiveness and simplicity of the underlying algebra.