We present a system which generates interactive high-level debugging systems from formal language definitions. The language definer has to specify a denotational semantics augmented with a formal description of the language specific debugging facilities. The generated debugger offers the traditional features such as tracing programs, setting breakpoints, displaying variables etc; interaction with the user is always on language level rather than on machine level. The concept has been implemented as part of the PSG-Programming System Generator, and has successfully been used to generate debuggers for Pascal and Modula-2. The core of the implementation consists of an interpreter for a functional language, which has been extended with the language-independent mechanisms needed in order to allow interaction with the user during program execution.

The PSG programming system generator developed at the Technical University of Darmstadt produces interactive, language-specific programming environments from formal language definitions. All language-dependent parts of the environment are generated from an entirely nonprocedural specification of the language's syntax, context conditions, and dynamic semantics. The generated environment consists of a language-based editor, supporting systematic program development by named program fragments, an interpreter, and a fragment library system. The major component of the environment is a full-screen editor, which allows both structure and text editing. In structure mode the editor guarantees prevention of both syntactic and semantic errors, whereas in textual mode it guarantees their immediate recognition. PSG editors employ a novel algorithm for incremental semantic analysis which is based on unification. The algorithm will immediately detect semantic errors even in incomplete program fragments. The dynamic semantics of the language are defined in denotational style using a functional language based on the lambda calculus. Program fragments are compiled to terms of the functional language which are executed by an interpreter. The PSG generator has been used to produce environments for Pascal, ALGOL 60, MODULA-2, and the formal language definition language itself.