Writer example using Dart

main.dart

import 'writer.dart';

void main() {
  final initial = '   mateus é pikão!!   '.writer;

  // Instead of calling functions on a String, we bind them
  // in our Writer<String>, building purely our log.
  final result =
    initial.bind(trim)
           .bind(toUpperCase);

  
  print('Result of transformations: ${result.value}');
  
  // Only after the log has been completely and purely built
  // we use them with side-effects
  print('Log: ${result.log}');
}

Writer<String> toUpperCase(String s) =>
    Writer(s.toUpperCase(), 'String \'$s\' was uppercased!');

Writer<String> trim(String s) => Writer(s.trim(), 'String \'$s\' was trimmed!');

writer.dart

import 'package:dartz/dartz.dart';


extension WriterStringExtension<T> on T {
  Writer<T> get writer => Writer(this, '');
}

class Writer<T> implements MonadOps<Writer, T> {
  final T value;
  final String log;

  Writer(this.value, this.log);

  @override
  Writer<T> bind<U>(Function1<T, Writer<T>> f) {
    final result = f(value);
    return Writer(
      result.value,
      '''$log
[${DateTime.now()}] ${result.log}''',
    );
  }

  @override
  Writer<B> map<B>(Function1<T, B> f) {
    final result = f(value);
    return Writer(result, log);
  }

  // TODO: Implement
  @override
  Writer<T> andThen<B>(covariant Writer<T> next) => throw UnimplementedError();

  @override
  Writer<T> ap<B>(covariant Writer<T> ff) => throw UnimplementedError();

  @override
  Writer<T> flatMap<B>(covariant Writer<T> Function(T a) f) =>
      throw UnimplementedError();

  @override
  Writer<T> replace<B>(B replacement) => throw UnimplementedError();

  @override
  Writer<T> strengthL<B>(B b) => throw UnimplementedError();

  @override
  Writer<T> strengthR<B>(B b) => throw UnimplementedError();
}