Generate lenses option

[laurenskz]

Lenses are a concept from functional programming that essentially focus a field of an object. A clear example is:

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

print(Person$.name.of(teacher).value);
// -> "Arthur"

print(Person$.age.of(teacher).update(60);
// -> Person(name: "Arthur", age: 60)

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

What is a lens. Essentially the lens interface is very very simple:

mixin Lens<T,K>{
   //Gets field from parent
   K get(T t);
   //Updates parent to set field to the value
   T update(T t,K k);

   Lens<T,C> then<C>(Lens<K,C> lens);
}

The beauty of lenses is that you can compose them. And the best thing is that you can use them with widgets in flutter. Suppose you have a state somewhere in your application, and in that state you have a list of some objects. You also have a widget that can update objects of that type. But your widget actually doesn't care where it sends the changes to. Lenses allow this abstraction. They just allow you to edit a value of the type you're interested in somewhere. Imagine a complicated nested object. Then you can for example say: Department$.manager.then(Manager$.employees).then(List$.index(0)).then(Employee$.salary). If you then combine the lens with a getter and setter for the source object (for example a Cubit) you can give widgets a place to put their modifications. This eliminates the need to say in all your Cubits : setFieldX(X x) => emit(state.copyWith(x : x).

Generating lenses is very simple, the only thing required would be generating a class as follows:

class ClassNameLenses{
  static Lens<ClassName<T>, fieldType> fieldName<T>() => Lens(
          (subject) => subject.fieldName,
          (subject, value) => subject.copyWith(fieldName: value));
}

I added type arguments for the case the class has them. The generation is simple and the feature would be very powerful

[rrousselGit]

I have no plan on generating lenses. I'd suggest using a separate code-generator for this instead