/* eslint-disable new-cap */
/* *************************************************************************************
* copyright: Copyright (c) 2021 Lowell D. Thomas, all rights reserved
* license: BSD-2-Clause (https://opensource.org/licenses/BSD-2-Clause)
* ********************************************************************************* */This example demonstrates how to call a UDT from a rule name callback function.
This is something you will probably never need to do and you might want to just skip over this example.
Using this feature changes the grammar that the parser will recognize in unpredictable ways,
unless you really, really know what you are doing and exactly how the UDT you are calling works.
The only reason these examples are here, is to verify that the
evaluateRule() and evaluateUdt() functions are working.
And the only reason this feature exists at all is because there was a case at one time where
I really wanted to use a UDT but I also really wanted some rule name nodes below it on the AST
for easy translation. For completeness, evaluateRule() was added along with evaluateUdt().
There are actually three demonstrations here.
UDT functions.All three demonstrations are done and the traces of them displayed on a single web page for comparison
of how the parser actually performed. For the basics of setting up and executing the parser
see setup.js.
(function colorsApp() {
const nodeUtil = require('node:util');
const inspectOptions = {
showHidden: true,
depth: null,
};
try {
let result;
let inputString;
let inputCharacterCodes;
let html;
const { apgLib } = require('apg-js');
const grammar = new (require('./colors'))();
const callbacks = require('./colors-callbacks');
const writeHtml = require('../writeHtml');
const fileName = 'colors-app';
const parser = new apgLib.parser();
/* set up the parser */
parser.trace = new apgLib.trace();
inputString = 'red,white,blue';
inputCharacterCodes = apgLib.utils.stringToChars(inputString);
const startRule = 0;
/* define the callback fuctions for demo one */
parser.callbacks.start = callbacks.startCallback;
parser.callbacks.u_blue = callbacks.u_blueCallback;
parser.callbacks.u_red = callbacks.u_redCallback;
parser.callbacks.u_white = callbacks.u_whiteCallback;
parser.callbacks.u_yellow = callbacks.u_yellowCallback;In this case, the color callback function just monitors the parser’s flow without altering it.
The input string is parsed and the trace saved for later display on a web page.
(See colors-callbacks.js for the callback functions.)
const data = [];
parser.callbacks.color = callbacks.monitorCallback;
result = parser.parse(grammar, startRule, inputCharacterCodes, data);
console.log();
console.log('the monitor callback results');
console.dir(result, inspectOptions);
html = parser.trace.toHtml(null, 'monitor callback');In this case, the color callback function actually modifies the parsing process
by returning a non-ACTIVE state. The action in this case is to always fail,
which is simply to demonstrate how modification from a syntax callback function can be done.
parser.callbacks.color = callbacks.modifyCallback;
result = parser.parse(grammar, startRule, inputCharacterCodes, data);
console.log();
console.log('the modify callback results');
console.dir(result, inspectOptions);
html += parser.trace.toHtml(null, 'modify callback');In this case the color callback function modifies the parsing process by calling a UDT function to do its work.
In addition to accepting the colors red, white and blue, it uses a UDT to accept yellow as well.
inputString = 'red,white,blue,yellow';
inputCharacterCodes = apgLib.utils.stringToChars(inputString);
parser.callbacks.color = callbacks.callUdtCallback;
result = parser.parse(grammar, startRule, inputCharacterCodes, data);
console.log();
console.log('the evaluateUdt() callback results');
console.dir(result, inspectOptions);
html += parser.trace.toHtml('ascii', 'evaluateUdt() callback');
html = apgLib.utils.htmlToPage(html, 'colors app');
writeHtml(html, fileName);
} catch (e) {
let msg = '\nEXCEPTION THROWN: \n';
if (e instanceof Error) {
msg += `${e.name}: ${e.message}`;
} else if (typeof e === 'string') {
msg += e;
} else {
msg += nodeUtil.inspect(e, inspectOptions);
}
process.exitCode = 1;
console.log(msg);
throw e;
}
})();