Functional lenses for Dart

Developing a Flutter app with BLoC? Check out the bloc_lens package for a more convenient way to manage complex app states!

Rationale

Sometimes, you want to allow other parts of your application to read and modify a certain value, but you don't want to expose the entire object or its internals.

In short, you want two operations:

• get the current value
• set a new value

This is where lenses come in. They encapsulate the value and provide a way to read and modify it, without exposing the object itself. THey are also composable, so you can easily modify e.g. a specific item in a list or an entry in a map.

Usage

Predefined lenses

This package provides a basic implementation of the Lens interface, called DirectLens. It allows you to combine a getter and a setter into a single object.

final lens = DirectLens(
  get: () => /* get the value */,
  set: (value) => /* set the value */,
);

Then, this object can be passed around and used to read and modify the value.

Specialized lenses

Aside from the standard Lens class, this package also offers more specializes classes, which provide additional utility methods:

• NumberLens – for (possibly bounded) numeric values.

  Configurable with optional minimum and maximum values, and an increment. Adds two methods: increment and decrement.

  NumberLens<int> lens;
  lens.get(); // 42
  lens.increment(); // 43
  lens.decrement(5); // 38
  

• BoolLens – for boolean values.

  Adds a toggle method.

  BoolLens lens;
  lens.get();    // true
  lens.toggle(); // false
  

• EnumLens – for enumerable values. Not just for actual enums, but for any type that has a finite number of values.

  Adds a next method, which cycles through the values.

  MyEnum { a, b, c }
  EnumLens<MyEnum> lens;
  lens.get();  // MyEnum.a
  lens.next(); // MyEnum.b
  

• ListLens – for lists of values.

  Adds methods for adding, removing, and modifying elements. It also provides a way to "look deeper" and get a lens for a specific element of the list.

  ListLens<int> lens;
  lens.get(); // [1, 2, 3]
  lens.add(4); // [1, 2, 3, 4]
  lens.removeAt(1); // [1, 3, 4]
  lens[1].set(100); // [1, 100, 4]
  

• MapLens – for maps of values.

  Adds methods for adding, removing, and modifying specific entries, as well as modifying all entries at once.

  MapLens<String, int> lens;
  lens.get(); // {'a': 1, 'b': 2}
  lens.updateAll((key, value) => value * 2); // {'a': 2, 'b': 4}
  lens['a'].set(42); // {'a': 42, 'b': 4}
  lens['c'].set(3); // {'a': 42, 'b': 4, 'c': 3}
  lens['b'].remove(); // {'a': 42, 'c': 3}
  

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