1Marpa::XS::Tracing(3) User Contributed Perl DocumentationMarpa::XS::Tracing(3)
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6 Marpa::XS::Tracing - Tracing your Marpa grammar
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9 This document is an overview of the techniques for tracing and
10 debugging Marpa parses and grammars.
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13 Check the input location where parsing failed
14 If parsing failed in the recognizer, look at the input location where
15 it happened. Compare the input against the grammar. This step is
16 fairly obvious, but I include it because even experts (actually,
17 especially experts) will sometimes overlook the obvious in a rush to
18 use more advanced techniques.
19 Turn on warnings
21 Make sure that Marpa's "warnings" named arguments for both the grammar
22 and the recognizer are turned on. Warnings are on by default.
23 Trace terminals
25 Turn on the "trace_terminals" recognizer named argument. This tells
26 you which tokens the recognizer is looking for and which ones it thinks
27 it found. If the problem is in lexing, "trace_terminals" tells you the
28 whole story.
29 Even if the problem is not in the lexing, tracing terminals can tell
31 you a lot. Marpa uses prediction-driven lexing. At any given parse
32 location, Marpa is only looking for those tokens that it thinks could
33 result in a successful parse. Examining the list of tokens that the
34 recognizer is looking for can also tell you where the recognizer thinks
35 it is.
36 Trace progress
38 Tracing the recognizer's progress with "show_progress" is most powerful
39 tool available in the basic toolkit. "show_progress" should provide
40 all the information necessary to debug an application's grammar. A
41 separate document explains how to interpret the progress reports. That
42 document includes an example of the use of "show_progress" to debug an
43 error in a grammar.
44 Double check rules and symbols
46 It sometimes helps to look carefully at the output of "show_rules" and
47 "show_symbols". Check if anything there is not what you expected.
48 Other traces
50 "trace_actions" will show you how action names resolve to actions.
51 Setting the "trace_values" evaluator named argument to a trace level of
52 1 traces the values of the parse tree nodes as they are pushed on, and
53 popped off, the evaluation stack.
54 Basic checklist
56 A full investigation of a parse includes the following:
57 · Make sure the "warnings" option is turned on. It is on by default.
59 · Turn on the "trace_terminals" recognizer named argument.
61 · Run "show_symbols" on the precomputed grammar.
63 · Run "show_rules" on the precomputed grammar.
65 · Run "show_progress" on the recognizer.
67 · Turn on the "trace_actions" evaluator named argument.
69 · Set the "trace_values" evaluator named argument to level 1.
71 When considering how much tracing to turn on, remember that if the
73 input text to the grammar is large, the outputs from "trace_terminals",
74 "show_progress", and "trace_values" will be very lengthy. You want to
75 work with short inputs if at all possible.
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78 Copyright 2012 Jeffrey Kegler
79 This file is part of Marpa::XS. Marpa::XS is free software: you can
80 redistribute it and/or modify it under the terms of the GNU Lesser
81 General Public License as published by the Free Software Foundation,
82 either version 3 of the License, or (at your option) any later version.
83 Marpa::XS is distributed in the hope that it will be useful,
85 but WITHOUT ANY WARRANTY; without even the implied warranty of
86 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
87 Lesser General Public License for more details.
88 You should have received a copy of the GNU Lesser
90 General Public License along with Marpa::XS. If not, see
91 http://www.gnu.org/licenses/.
92perl v5.30.1 2020-01-30 Marpa::XS::Tracing(3)