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.

Dump the parse value

Sometimes, even when there is a parse error, you can still evaluate the parse using the SLIF recognizer's "value()" method. If you are fortunate enough to have a parse value at the point of failure, it can be an excellent way to determine what the parser thinks it has seen so far. It is more likely that there will be a parse value if you are using incremental development, and the parse values will be especially helpful if your parse values are AST's.

If you are trying to run diagnostics on a failed parse, it is useful to catch the exception using "eval":

    my $eval_error = $EVAL_ERROR if not eval { $slr->read( \$test_input ); 1 };
    $progress_report = $slr->show_progress( 0, -1 );

Trace terminals

Set the "trace_terminals" recognizer named argument to 1. 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.

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_symbols" and "show_rules". Check if anything there is not what you expected. For thorough checking, it can be helpful to use a verbosity level higher than 1.

Other traces

Setting the SLIF recognizer's "trace_values" 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:

• Of course, the error message.
• If the failed parse returns a value, a dump of that value.
• Set the SLIF recognizer's "trace_terminals" named argument to level 1.
• Run "show_rules" on the SLIF grammar.
• Run "show_symbols" on the SLIF grammar.
• Run "show_progress()" on the SLIF recognizer.
• Set the SLIF recognizer's "trace_terminals" 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", and the dump of the parse value, can be very lengthy. You want to work with short inputs when possible.