union-type

union-type-js

A small JavaScript library for defining and using union types.

Union types are a way to group different values together. You can think of them as a powerful form of enums with the possibility to have additional data associated with the possible values.

Table of contents

• Tutorial
  • Defining a union type
  • Constructing a union type
  • Switching on union types
  • Extracting fields from a union type
  • Recursive union types
• Author & license

Tutorial

Defining a union type

union-type-js exports a single function Type. Union types are created by passing the Type function a definition object.

function isNumber(n) { return typeof n === 'number'; }
var Point = Type({Point: [isNumber, isNumber]});

The keys of the object are the names of the values that the type can have. The values of the object are arrays describing the fields of the value. The fields can be described by a validator function. When a value of the type is constructed the values passed to the constructor will have to pass the validator predicate.

Alternatively the fields can be specified by one of the standard built-in constructors Number, String, Object, Array or Function. union-type-js will detect these constructors and convert them to matching validator functions. Thus the above example is equivelant to this:

var Point = Type({Point: [Number, Number]});

Finally fields can be described in terms of other types.

var Shape = Type({
  Circle: [Number, Point],
  Rectangle: [Point, Point]
});

The values of a type can also have no fields at all.

var NotifySetting = Type({Mute: [], Vibrate: [], Sound: [Number]});

Constructing a union type

The Type function returns an object with constructor function for the different specified values. Thus, once you've defined a union type like this

var Point = Type({Point: [Number, Number]});
var Shape = Type({
  Circle: [Number, Point],
  Rectangle: [Point, Point]
});

You can create values like this:

var center = Point.Point(12, 7);
var radius = 8;
var circle = Shape.Circle(radius, center);

If you in any way pass a field value that does not match the specification a helpful error is thrown.

var p = Point.Point('bad', 4);
// throws TypeError: wrong value bad passed to location 0 in Point

Switching on union types

Every created type has a case function available along with its value constructors. case can be used as a control structure for handling the different values a type can have:

var Move = Type({Up: [], Right: [], Down: [], Left: []});

var player = {x: 0, y: 0};

var advancePlayer = function(move, player) {
  return Move.case({
    Up: function() { return {x: player.x, y: player.y - 1}; },
    Right: function() { return {x: player.x + 1, y: player.y}; },
    Down: function() { return {x: player.x, y: player.y + 1}; },
    Left: function() { return {x: player.x - 1, y: player.y}; },
  }, move);
};

Or with ECMAScript 6 syntax.

const advancePlayer = (move, player) =>
  Move.case({
    Up: () => ({x: player.x, y: player.y - 1}),
    Right: () => ({x: player.x + 1, y: player.y}),
    Down: () => ({x: player.x, y: player.y + 1}),
    Left: () => ({x: player.x - 1, y: player.y}),
  }, move);

case will extract the fields of a value and pass them in order to the relevant function. A function to calculate the area of a shape could, for instance, look like this.

var Shape = Type({Circle: [Number, Point],
                  Rectangle: [Point, Point]});
var area = (shape) =>
  Shape.case({
    Circle: (radius, _) => Math.PI * radius * radius,
    Rectangle: (p1, p2) => (p2[0] - p1[0]) * (p2[1] - p1[0])
  }, shape);

case is curried so we could have created the above function simply by not passing the second parameter to case.

var area = Shape.case({
  Circle: (radius, _) => Math.PI * radius * radius,
  Rectangle: (p1, p2) => (p2[0] - p1[0]) * (p2[1] - p1[0])
});

As a catch all you can supply a property with the key _ to case. When a type doesn't match another handler _ will be used.

const advancePlayerOnlyUp = (move, player) =>
  Move.case({
    Up: () => ({x: player.x, y: player.y - 1}),
    _: () => player,
  });

Extracting fields from a union type

The value of a union type is an array. This makes it easy to access the different fields.

var Person = Type({Person: [String, Number, Shape]});
var person = Person.Person('Simon', 21, Circle);
var name = person[0];
var age = person[1];
var favoriteShape = person[2];

Using the destructuring assignment in ECMAScript 6 it is possible to concisely extract all fields of a type.

var [name, age, favoriteShape] = person;

Recursive union types

It is possible to define recursive union types. In the example below, List is being used in it's own definition, thus it is still undefined when being passed to Type. Therefore Type interprets undefined as being a recursive invocation of the type currently being defined.

var List = Type({Nil: [], Cons: [R.T, List]});

We can write a function that recursively prints the content of our cons list.

var toString = List.case({
  Cons: (head, tail) => head + ' : ' + toString(tail),
  Nil: () => 'Nil',
});

var list = List.Cons(1, List.Cons(2, List.Cons(3, List.Nil())));
console.log(toString(list)); // => '1 : 2 : 3 : Nil'

Author & license

union-type-js was made by paldepind and is released under the MIT license. I hope you find it useful.