NAME

       Marpa::R2::NAIF::Tracing - Tracing a NAIF grammar

Description

       This document is an overview of the techniques for tracing and debugging Marpa parses and
       grammars.  using its named argument interface (NAIF).  If you are a beginner, or are not
       sure which interface you are interested in, or do not know what the NAIF interfaces is,
       you probably are looking for the document on tracing and debugging for the 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.

   Turn on warnings
       Make sure that Marpa's "warnings" named arguments for both the grammar and the recognizer
       are turned on.  Warnings are on by default.

   Trace terminals
       Turn on the "trace_terminals" recognizer named argument.  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.  Marpa
       uses prediction-driven lexing.  At any given parse location, Marpa is only looking for
       those tokens that it thinks could result in a successful parse.  Examining the list of
       tokens that the recognizer is looking for can tell you a lot about what the recognizer
       thinks it has seen so far.

   Trace progress
       Tracing the recognizer's progress with "show_progress" is 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_rules" and "show_symbols".
       Check if anything there is not what you expected.

   Other traces
       "trace_actions" will show you how action names resolve to actions.  Setting the
       "trace_values" evaluator 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:

       •   Make sure the "warnings" option is turned on.  It is on by default.

       •   Turn on the "trace_terminals" recognizer named argument.

       •   Run "show_symbols" on the precomputed grammar.

       •   Run "show_rules" on the precomputed grammar.

       •   Run "show_progress" on the recognizer.

       •   Turn on the "trace_actions" evaluator named argument.

       •   Set the "trace_values" evaluator named argument to level 1.

       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"
       will be very lengthy.  You want to work with short inputs if at all possible.

Copyright and License

         Copyright 2014 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/.