platform/packages/combinator/example/json.dart

import 'dart:io';
import 'package:angel3_combinator/angel3_combinator.dart';
import 'package:string_scanner/string_scanner.dart';

Parser jsonGrammar() {
  var expr = reference();

  // Parse a number
  var number = match<num>(RegExp(r'-?[0-9]+(\.[0-9]+)?'),
          errorMessage: 'Expected a number.')
      .value(
    (r) => num.parse(r.span!.text),
  );

  // Parse a string (no escapes supported, because lazy).
  var string =
      match(RegExp(r'"[^"]*"'), errorMessage: 'Expected a string.').value(
    (r) => r.span!.text.substring(1, r.span!.text.length - 1),
  );

  // Parse an array
  var array = expr
      .space()
      .separatedByComma()
      .surroundedBySquareBrackets(defaultValue: []);

  // KV pair
  var keyValuePair = chain([
    string.space(),
    match(':').space(),
    expr.error(errorMessage: 'Missing expression.'),
  ]).castDynamic().cast<Map>().value((r) => {r.value![0]: r.value![2]});

  // Parse an object.
  var object = keyValuePair
      .separatedByComma()
      .castDynamic()
      .surroundedByCurlyBraces(defaultValue: {});

  expr.parser = longest(
    [
      array,
      number,
      string,
      object.error(),
    ],
    errorMessage: 'Expected an expression.',
  ).space();

  return expr.foldErrors();
}

main() {
  var JSON = jsonGrammar();

  while (true) {
    stdout.write('Enter some JSON: ');
    var line = stdin.readLineSync()!;
    var scanner = SpanScanner(line, sourceUrl: 'stdin');
    var result = JSON.parse(scanner);

    if (!result.successful) {
      for (var error in result.errors) {
        print(error.toolString);
        print(error.span!.highlight(color: true));
      }
    } else
      print(result.value);
  }
}