Marpa::R2::Tracing - Tracing your grammar
This document is an overview of the techniques for tracing and debugging parses and grammars using Marpa's SLIF interface.
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.
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 );
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.
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.
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.
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.
A full investigation of a parse includes the following:
trace_terminals named argument to level 1.show_rules on the SLIF grammar.show_symbols on the SLIF grammar.show_progress() on the SLIF recognizer.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.
Copyright 2015 Jeffrey Kegler This file is part of Marpa::R2. Marpa::R2 is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. Marpa::R2 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. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with Marpa::R2. If not, see http://www.gnu.org/licenses/.
