Lists, Optional Elements, and Placeholders

Abstract

Lists and optional elements are such an important aspect of editor specifications that they deserve to be discussed in more detail. Although iterated elements (lists) and optional elements are concepts that are commonly found in many extended BNF notations, the form that they take in the Synthesizer Specification Language is different from the familiar notions. The chief motivation for SSL’s notation for lists and optional elements is to provide a uniform notation for specifying the attribution of terms and their unparsing. The attribute equations and, with one small exception, the unparsing declarations that one writes for list phyla and optional phyla are no different in form from those that are used for ordinary phyla. For example, because all lists are terminated by an instance of the list phylum’s nullary operator, there is no need for special-case rules covering the cases of an empty list, a singleton list, and a list of length two or more, as would otherwise have been necessary.