Lightweight and generative components I: Source-level components

Abstract. Current denitions of \software component " are based on abstract data types | collections of functions together with local data. This paper addresses two ways in which this denition is inadequate: it fails to allow for lightweight components | those for which a function call is too inecient or semantically inappropriate | and it fails to allow for generative components | those in which the component embodies a method of constructing code rather than actual code. We argue that both can be solved by proper use of existing language technologies, by using a higher-order meta-language to compositionally manipulate values of type Code, syntactic fragments of some object language. By dening a client as a function from a component to Code, components can be dened at a very general level without much notational overhead. In this paper, we illustrate this idea entirely at the source-code level, taking Code to be string. Operating at this level is particularly simple, and is useful when the source code is not proprietary. In a companion paper, we dene Code as a set of values containing machine-language code (as well as some additional structure), allowing components to be delivered in binary form.