List Processing and Extension of Language Facility by Embedding

There are two distinct (though not necessarily disjoint) types of language extension. The first is a syntactic extension, in which the class of expressions interpretable within a language is widened. The second is an extension of the functional domain in which operations not previously performable are added to the language. These extensions may be implemented by completely reprogramming the basic language processor (the compiler or interpreter), but such extensions may often be achieved at far less cost by embedding. The authors give an example of each of these two types of extensions by embedding, adding list-processing facilities to the FORTRAN language, and extending the list processing language LISP to facilitate expression and interpretation of string transformations. As an aid to those unfamiliar with list processing, there is a section which reviews this subject. Included are descriptions and explanations of list structures, the various notations used for lists, the basic processes of list processing, and the different notations (machine language, functional, and prototype) used for expressing processing of lists.