++ed by:

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Jeffrey Kegler
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Marpa::R3::Tracing - Tracing your grammar


This document is an overview of the techniques for tracing and debugging parses and grammars using Marpa's SLIF interface.

Basic techniques

Check the input location where parsing failed

If parsing failed in the recognizer, look at the input location where it happened. Compare the input against the grammar. This step is fairly obvious, but I include it because even experts (actually, especially experts) will sometimes overlook the obvious in a rush to use more advanced techniques.

Dump the parse value

Sometimes, even when there is a parse error, you can still evaluate the parse using the SLIF recognizer's value() method. If you are fortunate enough to have a parse value at the point of failure, it can be an excellent way to determine what the parser thinks it has seen so far. It is more likely that there will be a parse value if you are using incremental development, and the parse values will be especially helpful if your parse values are AST's.

If you are trying to run diagnostics on a failed parse, it is useful to catch the exception using eval:

    my $eval_error = $EVAL_ERROR if not eval { $recce->read( \$test_input ); 1 };
    $progress_report = $recce->show_progress( 0, -1 );

Trace terminals

Set the trace_terminals recognizer named argument to 1. This tells you which tokens the recognizer is looking for and which ones it thinks it found. If the problem is in lexing, trace_terminals tells you the whole story. Even if the problem is not in the lexing, tracing terminals can tell you a lot.

Trace progress

Tracing the recognizer's progress with show_progress is the most powerful tool available in the basic toolkit. show_progress should provide all the information necessary to debug an application's grammar. A separate document explains how to interpret the progress reports. That document includes an example of the use of show_progress to debug an error in a grammar.

Double check rules and symbols

It sometimes helps to look carefully at the output of show_symbols and show_rules. Check if anything there is not what you expected. For thorough checking, it can be helpful to use a verbosity level higher than 1.

Other traces

Setting the SLIF recognizer's trace_values named argument to a trace level of 1 traces the values of the parse tree nodes as they are pushed on, and popped off, the evaluation stack.

Basic checklist

A full investigation of a parse includes the following:

When considering how much tracing to turn on, remember that if the input text to the grammar is large, the outputs from trace_terminals, show_progress, and trace_values, and the dump of the parse value, can be very lengthy. You want to work with short inputs when possible.


  Marpa::R3 is Copyright (C) 2017, Jeffrey Kegler.

  This module is free software; you can redistribute it and/or modify it
  under the same terms as Perl 5.10.1. For more details, see the full text
  of the licenses in the directory LICENSES.

  This program is distributed in the hope that it will be
  useful, but without any warranty; without even the implied
  warranty of merchantability or fitness for a particular purpose.