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.
David Majda
10 years ago