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.IX Title "Marpa::R2::NAIF::Tracing 3pm"
.TH Marpa::R2::NAIF::Tracing 3pm "2021-01-22" "perl v5.32.0" "User Contributed Perl Documentation"
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.SH "NAME"
Marpa::R2::NAIF::Tracing \- Tracing a NAIF grammar
.SH "Description"
.IX Header "Description"
This document is an overview of
the techniques for tracing and
debugging Marpa parses and grammars.
using its named argument interface (\s-1NAIF\s0).
If you are a beginner,
or are not sure which interface you are interested in,
or do not know what the \s-1NAIF\s0 interfaces is,
you probably are looking for
the document on tracing and debugging for the \s-1SLIF\s0
interface.
.SH "Basic techniques"
.IX Header "Basic techniques"
.SS "Check the input location where parsing failed"
.IX Subsection "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.
.SS "Turn on warnings"
.IX Subsection "Turn on warnings"
Make sure that
Marpa's \f(CW\*(C`warnings\*(C'\fR named arguments
for both the grammar and the recognizer
are turned on.
Warnings are on by default.
.SS "Trace terminals"
.IX Subsection "Trace terminals"
Turn on the \f(CW\*(C`trace_terminals\*(C'\fR 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, \f(CW\*(C`trace_terminals\*(C'\fR tells you the whole story.
.PP
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.
.SS "Trace progress"
.IX Subsection "Trace progress"
Tracing the recognizer's progress
with
\&\f(CW\*(C`show_progress\*(C'\fR
is most powerful tool available
in the basic toolkit.
\&\f(CW\*(C`show_progress\*(C'\fR
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 \f(CW\*(C`show_progress\*(C'\fR
to debug an error in a grammar.
.SS "Double check rules and symbols"
.IX Subsection "Double check rules and symbols"
It sometimes helps to look carefully at the output of
\&\f(CW\*(C`show_rules\*(C'\fR and \f(CW\*(C`show_symbols\*(C'\fR.
Check if anything there is
not what you expected.
.SS "Other traces"
.IX Subsection "Other traces"
\&\f(CW\*(C`trace_actions\*(C'\fR will show you how action names resolve to actions.
Setting the \f(CW\*(C`trace_values\*(C'\fR 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.
.SS "Basic checklist"
.IX Subsection "Basic checklist"
A full investigation of a parse
includes the following:
.IP "\(bu" 4
Make sure the \f(CW\*(C`warnings\*(C'\fR option is turned on.  It is on by default.
.IP "\(bu" 4
Turn on the \f(CW\*(C`trace_terminals\*(C'\fR recognizer named argument.
.IP "\(bu" 4
Run \f(CW\*(C`show_symbols\*(C'\fR on the precomputed grammar.
.IP "\(bu" 4
Run \f(CW\*(C`show_rules\*(C'\fR on the precomputed grammar.
.IP "\(bu" 4
Run \f(CW\*(C`show_progress\*(C'\fR on the recognizer.
.IP "\(bu" 4
Turn on the \f(CW\*(C`trace_actions\*(C'\fR evaluator named argument.
.IP "\(bu" 4
Set the \f(CW\*(C`trace_values\*(C'\fR evaluator named argument to level 1.
.PP
When considering how much tracing to turn on,
remember that if the input text to the grammar is large,
the outputs from
\&\f(CW\*(C`trace_terminals\*(C'\fR,
\&\f(CW\*(C`show_progress\*(C'\fR,
and \f(CW\*(C`trace_values\*(C'\fR
will be very lengthy.
You want to work with short inputs if at all possible.
.SH "Copyright and License"
.IX Header "Copyright and License"
.Vb 5
\&  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/.
.Ve