Programming language description languages: From Christopher Strachey to semantics online

Since the middle of the twentieth century, hundreds of programming languages have been designed and implemented – and new ones are continually emerging. The syntax of a programming language can usually be described quite precisely and efficiently using formal grammars. However, the formal description of its semantics is much more challenging. Language designers, implementers and programmers commonly regard formal semantic descriptions as impractical. Research in semantics has allowed us to reason about software and has provided valuable insight into the design of programming languages, but few semantic descriptions of full languages have been published, and hardly any of these are currently available online.One of the major approaches to formal semantics is denotational semantics, which originated from Strachey’s pioneering studies in the 1960s. Why is such a theoretically attractive approach generally regarded as impractical for describing full-scale programming languages? Does it help much to use monads in denotational descriptions, or is a more radical change needed? How might efficient online access to a repository of semantic descriptions be provided? Could it ever become as easy to generate efficient compilers and interpreters from semantic descriptions as it already is to generate parsers from grammars? This chapter addresses such questions and gives some grounds for optimism about the development of highly practical, online semantic descriptions.