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David Majda da57118a43 Implement basic support for tracing
Parsers can now be generated with support for tracing using the --trace
CLI option or a boolean |trace| option to |PEG.buildParser|. This makes
them trace their progress, which can be useful for debugging. Parsers
generated with tracing support are called "tracing parsers".

When a tracing parser executes, by default it traces the rules it enters
and exits by writing messages to the console. For example, a parser
built from this grammar:

  start = a / b
  a = "a"
  b = "b"

will write this to the console when parsing input "b":

  1:1 rule.enter start
  1:1 rule.enter   a
  1:1 rule.fail    a
  1:1 rule.enter   b
  1:2 rule.match   b
  1:2 rule.match start

You can customize tracing by passing a custom *tracer* to parser's
|parse| method using the |tracer| option:

  parser.parse(input, { trace: tracer });

This will replace the built-in default tracer (which writes to the
console) by the tracer you supplied.

The tracer must be an object with a |trace| method. This method is
called each time a tracing event happens. It takes one argument which is
an object describing the tracing event.

Currently, three events are supported:

  * rule.enter -- triggered when a rule is entered
  * rule.match -- triggered when a rule matches successfully
  * rule.fail  -- triggered when a rule fails to match

These events are triggered in nested pairs -- for each rule.enter event
there is a matching rule.match or rule.fail event.

The event object passed as an argument to |trace| contains these
properties:

  * type   -- event type
  * rule   -- name of the rule the event is related to
  * offset -- parse position at the time of the event
  * line   -- line at the time of the event
  * column -- column at the time of the event
  * result -- rule's match result (only for rule.match event)

The whole tracing API is somewhat experimental (which is why it isn't
documented properly yet) and I expect it will evolve over time as
experience is gained.

The default tracer is also somewhat bare-bones. I hope that PEG.js user
community will develop more sophisticated tracers over time and I'll be
able to integrate their best ideas into the default tracer.
9 years ago
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pegjs Implement basic support for tracing 9 years ago