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.