Ambiguity Detection Methods for Context-Free Grammars

The Meta-Environment enables the creation of grammars using the SDF formalism. From these grammars an SGLR parser can be generated. One of the advantages of these parsers is that they can handle the entire class of context-free grammars (CFGs). The grammar developer does not have to squeeze his grammar into a specific subclass of CFGs that is deterministically parsable. Instead, he can now design his grammar to best describe the structure of his language. The downside of allowing the entire class of CFGs is the danger of ambiguities. An ambiguous grammar prevents some sentences from having a unique meaning, depending on the semantics of the used language. It is best to remove all ambiguities from a grammar before it is used. Unfortunately, the detection of ambiguities in a grammar is an undecidable problem. For a recursive grammar the number of possibilities that have to be checked might be infinite. Various ambiguity detection methods (ADMs) exist, but none can always correctly identify the (un)ambiguity of a grammar. They all try to attack the problem from different angles, which results in different characteristics like termination, accuracy and performance. The goal of this project was to find out which method has the best practical usability. In particu

Reachability and Error Diagnosis in LR(1) Parsers

International audienceGiven an LR(1) automaton, what are the states in which an error can be detected? For each such " error state " , what is a minimal input sentence that causes an error in this state? We propose an algorithm that answers these questions. This allows building a collection of pairs of an erroneous input sentence and a (handwritten) diagnostic message, ensuring that this collection covers every error state, and maintaining this property as the grammar evolves. We report on an application of this technique to the CompCert ISO C99 parser, and discuss its strengths and limitations

On Designing Programming Error Messages for Novices: Readability and its Constituent Factors

The 2021 ACM CHI Virtual Conference on Human Factors in Computing Systems (CHI'21), Virtual Conference, 8-13 May 2021Programming error messages play an important role in learning to program. The cycle of program input and error message response completes a loop between the programmer and the compiler/interpreter and is a fundamental interaction between human and computer. However, error messages are notoriously problematic, especially for novices. Despite numerous guidelines citing the importance of message readability, there is little empirical research dedicated to understanding and assessing it. We report three related experiments investigating factors that influence programming error message readability. In the first two experiments we identify possible factors, and in the third we ask novice programmers to rate messages using scales derived from these factors. We find evidence that several key factors significantly affect message readability: message length, jargon use, sentence structure, and vocabulary. This provides novel empirical support for previously untested long-standing guidelines on message design, and informs future efforts to create readability metrics for programming error messages

Automatic error recovery for LR parsers in theory and practice

This thesis argues the need for good syntax error handling schemes in language translation systems such as compilers, and for the automatic incorporation of such schemes into parser-generators. Syntax errors are studied in a theoretical framework and practical methods for handling syntax errors are presented. The theoretical framework consists of a model for syntax errors based on the concept of a minimum prefix-defined error correction,a sentence obtainable from an erroneous string by performing edit operations at prefix-defined (parser defined) errors. It is shown that for an arbitrary context-free language, it is undecidable whether a better than arbitrary choice of edit operations can be made at a prefix-defined error. For common programming languages,it is shown that minimum-distance errors and prefix-defined errors do not necessarily coincide, and that there exists an infinite number of programs that differ in a single symbol only; sets of equivalent insertions are exhibited. Two methods for syntax error recovery are, presented. The methods are language independent and suitable for automatic generation. The first method consists of two stages, local repair followed if necessary by phrase-level repair. The second method consists of a single stage in which a locally minimum-distance repair is computed. Both methods are developed for use in the practical LR parser-generator yacc, requiring no additional specifications from the user. A scheme for the automatic generation of diagnostic messages in terms of the source input is presented. Performance of the methods in practice is evaluated using a formal method based on minimum-distance and prefix-defined error correction. The methods compare favourably with existing methods for error recovery

An Estelle compiler

The increasing development and use of computer networks has necessitated international standards to be defined. Central to the standardization efforts is the concept of a Formal Description Technique (FDT) which is used to provide a definition medium for communication protocols and services. This document describes the design and implementation of one of the few existing compilers for the one such FDT, the language "Estelle" ([ISO85], [ISO86], [ISO87])