The Term Graph Programming System HOPS

Wolfram Kahl Universit at der Bundeswehr M unchen, D-85577 Neubiberg, Germany 1 Motivation Programmers of Haskell or ML know that once their programs are syntactically correct and get past the type checker, the probability that they are correct is usually already pretty high. But while the parser or the type checker still complain, it is not always easy to spot the real problem, which may be far from the trouble spot indicated by the often cryptic error message. Syntax-directed editing can solve the problem with syntax errors. But, even if a syntax-directed editor would be extended with online typing, another problem would be how to present the typing information to the user; huge type expressions with unclear relation to the program expression may perhaps be of only very little help. We present a system that deals with both problems by making the program structure and the program's typing structure explicit and interactively accessible in the shape of term graphs. The Higher Object Programming System HOPS, which has been developed by a group led by Gunther Schmidt since the mid-eighties [ZSB86,Kah94], is a graphically interactive term graph editing and transformation system designed for transformational program development. All drawings in this not have been generated from the term graph interface of the latest version of HOPS. 2 Principles of HOPS HOPS manipulates arbitrary second-order term graphs, where all the structure usually encoded via name and scope is made explicit. Term graphs in HOPS therefore feature nameless variables, explicit variable binding , explicit variable identity (to denote which nodes stand for the same variables) and metavariables with arbitrary arity. All the problems usually connected with name clashes and variable renaming are therefore avoided. Consider the following examples: + × 2 × 1 λ