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Author image Jeffrey Kegler
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Marpa::Tracing - Tracing Your Grammar


This document describes techniques for tracing and debugging Marpa parses and grammars.

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. The warnings named arguments are on by default.

Turn off Stripping

When Marpa "strips" its objects, it removes data that is not needed for subsequent processing. This saves time and memory, but data that is not needed for processing can be extremely valuable for debugging. When objects are stripped, many of Marpa's tracing methods will return partial information or no information at all.

You should turn off Marpa's strip named argument to the grammar. Grammar stripping is on by default. If you are using the recognizer's strip method to strip the recognizer, stop doing that.

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. That means that the list of tokens that the recognizer is looking for tells you where the recognizer thinks it is.

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 semantic Perl closures. 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.

You can trace the earley sets as they are completed, but it is usually best to wait until input to the recognizer is finished. At that point call Marpa::Recognizer::show_earley_sets . Since the items in the earley sets use QDFA states, you usually will want to call Marpa::Grammar::show_QDFA as well. The implementation document has example outputs from the show_earley_sets and show_QDFA methods, and explains how to read them.


A complete investigation of a parse includes the following:

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

  • Turn off the strip Marpa named argument. By default, it is on.

  • Make sure you're not stripping the recognizer.

  • Turn on the trace_terminals recognizer named argument.

  • Run show_symbols on the precomputed grammar.

  • Run show_rules on the precomputed grammar.

  • Turn on the trace_actions evaluator named argument.

  • Set the trace_values evaluator named argument to level 1.

  • Run show_earley_sets on the recognizer.

  • Run show_QDFA on the precomputed grammar.

When considering how much tracing to turn on, remember that if the input text to the grammar is large, the outputs from show_earley_sets, trace_terminals, and trace_values can be very lengthy. You want to work with short inputs if at all possible.


Copyright 2007-2010 Jeffrey Kegler, all rights reserved. Marpa is free software under the Perl license. For details see the LICENSE file in the Marpa distribution.