VM only

• Uses dart:io File.

Step 1 - Line stream

import 'dart:convert';
import 'dart:io';

Stream<String> linesOf(File file) {
  return file.openRead().transform(utf8.decoder).transform(const LineSplitter());
}

Step 2 - Process with await for

import 'dart:convert';
import 'dart:io';

Future<void> main() async {
  final f = File('sample.txt');
  await f.writeAsString('one\ntwo\nthree\n');
  await for (final line in linesOf(f)) {
    if (line.trim().isEmpty) continue;
    print('line: $line');
  }
}

Stream<String> linesOf(File file) {
  return file.openRead().transform(utf8.decoder).transform(const LineSplitter());
}

Step 3 - Transform pipeline

import 'dart:convert';
import 'dart:io';

Future<void> main() async {
  final f = File('sample.txt');
  await f.writeAsString('alpha\nBETA\nGamma\n');
  final upperCount = await linesOf(f)
      .map((s) => s.trim())
      .where((s) => s.isNotEmpty)
      .where((s) => s == s.toUpperCase())
      .fold<int>(0, (c, _) => c + 1);
  print(upperCount);
}

Stream<String> linesOf(File file) {
  return file.openRead().transform(utf8.decoder).transform(const LineSplitter());
}

Practice tasks

• Count lines matching a prefix using where and fold<int>(0, (c, _) => c + 1) without collecting a List.
• On FileSystemException, print a friendly message and exit with non-zero code from main.
• Split work: first stream that only validates UTF-8, then map to a small record (int lineNo, String text) with asyncMap or an indexed helper.