Awesome Open Source
Awesome Open Source


Simple and non-intrusive code generator for boilerplate of data types. The package generates a simple mixin with operator==, hashCode, copyWith, toString, as well as lenses.

Boiler plate

Because the boiler plate is generated as a mixin, it is minimally intrusive on the interface of the class. You only have to provide a constructor with named arguments for all fields and extend the generated mixin.

class Person extends $Person {
  final String name;
  final int age;
  const Person({, this.age});
  const Person.anonymous() : this(name: "John Doe", age: null);
  int get ageInDays => numberOfDaysInMostYears * age;

  static const numberOfDaysInMostYears = 356;

Because of this design, you have complete control over the class. You can, for example, add named constructors or methods to the class like you're used to.


For every class, lenses are generated for all fields which allow viewing a field or creating a new instance of the classes with that field modified in some way. For example, the lens of Person's name is Person$.name. To focus a lens on a specific instance use its of method:

final teacher = Person(name: "Arthur", age: 53);

// -> "Arthur"

// -> Person(name: "Arthur", age: 60)

print(Person$.name.of(teacher).map((name) => name.toUpperCase()));
// -> Person(name: "ARTHUR", age: 53)

This isn't very exciting yet. The power of lenses comes to light when you combine them. It allows you to easily create a copy of a large nested data structure with one of the fields in a leaf modified. Two lenses can be chained using then.

class Course extends $Course {
  final String name;
  final List<Person> students;
  const Course({this.students});

final programming = Course(name: "Programming 101", students: [Person(name: "Jane", age: 21), Person(name: "Tom", age: 20)]);

final firstStudentsName = Course$.students.then(List$.first<Person>()).then(Person$.name);

// -> Course(students: [Person(name: "Marcy", age: 21), Person(name: "Tom", age: 20)]

Compare this with the alternative:

final firstStudent = programming.students.first;
final updatedFirstStudent = Person(name: "Marcy", age: firstStudent.age);
final updatedStudents = [updatedFirstStudent] + programming.students.skip(1);
final updatedCourse = Course(name:, students: updatedStudents);

This is much less readable and error prone. Imagine what happens when one of the classes gains a field.

Full example:

// lens.dart
import 'package:collection/collection.dart';

import 'package:functional_data/functional_data.dart';

part 'lens.g.dart';

// Only requirement is that it has a constructor with named arguments for all fields
class Foo extends $Foo {
  final int number;
  final String name;

  const Foo({this.number,});

class Bar extends $Bar {
  final Foo foo;

  final List<Foo> foos;

  final String driver;

  const Bar({, this.foos, this.driver});

void main() {
  final foo = Foo(number: 42, name: "Marvin");
  final bar = Bar(foo: foo, foos: [Foo(number: 101, name: "One"), Foo(number: 102, name: "Two")], driver: "One");

  final fooName = Bar$.foo.then(Foo$.name);
  // print( => name.toUpperCase(), bar));
  print(fooName.of(bar).map((name) => name.toUpperCase()));
  // Bar(foo: Foo(number: 42, name: MARVIN), foos: [Foo(number: 101, name: One), Foo(number: 102, name: Two)], driver: One)

  final firstFooName = Bar$.foos.then(List$.atIndex<Foo>(0)).then(Foo$.name);
  // print(firstFooName.update(bar, "Twee"));
  // Bar(foo: Foo(number: 42, name: Marvin), foos: [Foo(number: 101, name: Twee), Foo(number: 102, name: Two)], driver: One)

  final nameOfFooNamedTwo = Bar$.foos.then(List$.where<Foo>((foo) => == "Two")).then(Foo$.name);
  print(nameOfFooNamedTwo.update(bar, "Due"));
  // Bar(foo: Foo(number: 42, name: Marvin), foos: [Foo(number: 101, name: One), Foo(number: 102, name: Due)], driver: One)

  final driversNumber =
      Bar$.foos.thenWithContext((bar) => List$.where<Foo>((foo) => == bar.driver).then(Foo$.number));
  // 101

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