Interpreters From Functions and Grammars

-The addition of context free grammar rules to a functional language simplifies the construction of interpreters from denotationaI semantic language definitions. Functional abstraction over grammar rules enables the specification and processing of context sensitive language syntax aspects in a functional style. DenotatLonaI semantics Language design Language implementation Functional language Grammar rules Interpreters Context sensitive processing 1. I N T R O D U C T I O N 1.1 Denotational semantics Denotational semantics [1] is a formalism for defining programming languages. A denotational semantic definition consists of semantic domains, abstract syntax and semantic functions. The semantic domains define the universe of objects in which the language is given meaning, the abstract syntax defines meaningful syntactic constructs and the meaning of each abstract syntax construct is defined by a semantic function over the serriantic domains. Abstract syntax is distinguished from concrete syntax which describes the physical representation of programs and contains details which are not significant from a semantic point of view [2]. Approaches based on denotational semantics are used to provide succinct and theoretically rigorous programming language standards. These then inform language implementations and program manipulation. Such approaches are also used for programming language design [3] but there are few software tools which are appropriate for experimenting with denotational semantic based designs. 1.2 Compiler generators Compiler generators [4], which are used to automate the construction of production compilers, are based on very different methodologies to denotational semantics and place a far greater emphasis on concrete syntax. A typical compiler generator, like YACC [5], inputs a formalised definition of the context free [6] concrete syntax of a language along with semantic actions written in an imperative language. These implement context sensitive concrete syntax checking and the generation of the target language. When compiler generators are used with denotational semantic designs a basic problem is the lack of correspondence between the functional meta-language used to define semantic equations and the imperative semantic action language. Translating semantic equations into semantic actions loses clarity and is a potential source of errors. When the design changes so must the translation for the compiler generator. One alternative is to use an existing compiler generator with a new semantic action language which has a well defined correspondence with the functional meta-language [7]. Another is to develop a system which is oriented specifically for use with denotational semantics. 1.3 SIS SIS [8] is a language implementation system which enables direct experimentation with denotational semantic language designs. It takes in a full denotational semantic definition and uses it to generate what is in effect the code for an abstract machine. The code is then interpreted. The strict adherence to denotational semantics, however, with the formal separation of concrete syntax, abstract syntax and semantics, appears to restrict the ease with which SIS may be used.