The syntax definition formalism SDF—reference manual—

1989, ACM SIGPLAN Notices

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69 pages

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Abstract

SDF is a formalism for the definition of syntax which is comparable to BNF in some respects, but has a wider scope in that it also covers the definition of lexical and abstract syntax. Its design and implementation are tailored towards the language designer who wants to develop new languages as well as implement existing ones in a highly interactive manner. It emphasizes compactness of syntax definitions by offering (a) a standard interface between lexical and context-free syntax; (b) a standard correspondence between context-free and abstract syntax; (c) powerful disambiguation and list constructs; and (d) an efficient incremental implementation which accepts arbitrary context-free syntax definitions. SDF can be combined with a variety of programming and specification languages. In this way these obtain fully general user-definable syntax.

Figures (36)

These priorities select parses with the ordinary priority of + and * and a preference for functions on N. Figure 6.9 shows the three possible parse trees for the sentence 1.5+2*3. The priorities from the example are now applied as follows. Tree (a) contains a conflict since a node corre- sponding to * on R has as child a node corresponding to + on R. Trees (b) and (c) are conflict- free. Comparing these two trees, one sees that (c) >> (b) holds. The only function that occurs more often in (c) than in (b) is * on N: it occurs once in (c) but not in (b). This is compensated for by the fact that * on R (which has a lower priority) appears once in (b) but not in (c). Hence, tree (c) is the (only) parse tree associated with the sentence 1.5+2*3. As a final example, we show in Figure 6.10 how the interaction between general context-free

Figure 7.1. Declarations extended with variables

‘igure 7.2. (b) Abstract syntax tree for decl Id, c, Ids1 : Type. naming-scheme -> SORT,

Figure 7.2 shows the parse tree and the corresponding abstract syntax tree for the sentence de Id, c, Idsl : Type, which contains occurrences of the variables Id, Idsi and Type. Nc that variables, unlike lexical items, do not have any further structure in the abstract syntax tree.

11.4. Related work There have been many attempts to introduce user-definable syntax in programming languages. These ideas have led to user-definable syntax for operators in Algol68, Prolog, Snobol4 and other programming languages, and to various styles of macro-definitions (PL/I, Lisp). Around 1970, there was much interest in so-called extensible languages [Iro70, Sta75, Weg70]. The aim of this line of research was to define a small base language in combination with a syntax defini- tion formalism. New language constructs could then be added to the base language by defining their syntax and by describing their semantics in terms of the base language. For various reasons, however, the overall goal of full syntactic and semantic language extensibility has never been completely achieved. Although it does not have a syntactic extension mechanism, Smalltalk-80 may be viewed as the most successful extensible language in existence. (Smalltalk-72 — a prede- cessor of Smalltalk-80 — did support syntactic extensibility: each class had to parse the messages send to it explicitly. This feature has been replaced by a more limited scheme of keyword param- eters in Smalltalk-80. This new scheme result in more readable programs and allows a more effi- cient implementation.) a. ea a ee ee .. ee me ere ee ee ee ee, ee ET . Len eee