The thesis of this dissertation is that formal definitions of the syntax and semantics of computer languages are needed. This dissertation investigates two candidates for formally defining computer languages: (1) the formalism of canonical systems for defining the syntax of a computer language and its translation into a target language, and (2) the formalisms of the l-calculus and extended Markov algorithms as a combined formalism used as the basis of a target language for defining the semantics of a computer language.

The original aim of the work that led to this dissertation was to extend an existing, purely functional language with facilities for input/output and modular programming. The language is based on an untyped -calculus, i.e., program execution is defined as program transformation according to a fixed set of reduction rules including fi-reduction. Consistently, the implementation comprises an interactive reduction system which is integrated with a syntax-oriented editor: any sub-expression or program result can be submitted for (stepwise) reduction. There is no distinguished main program, no `global' environment and no explicit static part of the language -- in particular, there is no static type system. It is therefore not clear how to add one of the known solutions for input/output or modular programming to such a programming environment. Furthermore, simply adding features to the language would lead to a complex language design with weakly integrated parts, thus losing much of the appe...