S.41-50

We present PERPLEX, a programming environment intended for the end-user. In its design, the concepts of logic programming and spreadsheets are combined. Thus, on the one hand, logic programming becomes an interactive, incremental task where the user gets direct visual feedback, on the other hand, functionality and scope of a conventional spreadsheet program are considerably extended. In order to perform calculations and queries, constraints are imposed on the contents of the spreadsheet cells. New predicates can be defined using a programming-by-example technique: Rules are extracted from the user's solutions for example problems. Thus, concrete intermediate results take over the role of abstract logic variables in the programming process. PERPLEX has been successfully implemented on a Symbolics Lisp Machine.

Abstract An efficient and systematic LL(1) error recovery method is presented that has been implemented for an LL(1) parser generator. Error messages which provide good diagnostic information are generated automatically. Error correction is done by discarding some input symbols and popping up some symbols from the parsing‐stack in order to restore the parser to a valid configuration. Thus, symbol deletions and insertions are simulated. The choice between different possible corrections is made by comparing the cost of the inserted (popped) symbols with the reliability value of the recovery symbol (the first input symbol that is not discarded). Our concept of reliability is based on the observation that input symbols differ from each other in their ability to serve as recovery points. A high reliability value of a symbol asserts that it was probably not placed in the input by accident. So it is reasonable not to discard that symbol but to resume parsing. This is done even if a string with high insert‐cost has to be inserted before that symbol in order to fit it to the part of the program that has already been analysed. The error recovery routine is invoked only when an error is detected. Thus, there is no additional time required for parsing correct programs. Error‐correcting parsers for different languages, including Pascal, have been generated. Some experimental results are summarized.