union-type

union-type

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 exports a single function Type. Union types are created by passing the Type function a definition object. The easiest way to define a Type is as follows:

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 will detect these constructors and convert them to matching validator functions. Thus the above example is equivalent to this:

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

Records

Instead of supplying only the types of the individual constructors it is also possible to define records using object descriptions:

var Point = Type({Point: {x: Number, y: Number}});

Instance methods

Furthermore it is possible to add instance methods. A Maybe type with a map function could thus be defined as follows:

var T = function () { return true; };
var Maybe = Type({Just: [T], Nothing: []});
Maybe.prototype.map = function(fn) {
  return Maybe.case({
    Nothing: () => Maybe.Nothing,
    Just: (v) => Maybe.Just(fn(v))
  }, this);
};
var just = Maybe.Just(1);
var nothing = Maybe.Nothing;
nothing.map(add(1)); // => Nothing
just.map(add(1)); // => Just(2)

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('foo', 4);
// throws TypeError: bad value 'foo' passed to first argument of constructor Point

As mentioned earlier you can also define records using object descriptions:

var Point = Type({Point: {x: Number, y: Number}});

Types defined using the record syntax have to be constructed using the respective <name>Of constructor. The Point type above is hence constructed using PointOf:

var p = Point.PointOf({x: 1, y: 1});

Alternatively records can be constructed in the same way as regular types.

var p = Point.Point(1, 1);

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 Action = Type({Up: [], Right: [], Down: [], Left: [], Jump: [], Fire: [Number]});

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

var advancePlayer = function(action, player) {
  return Action.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}; },
    _: function() { return player; }
  }, action);
};

Or with ECMAScript 6 syntax.

const advancePlayer = (action, player) =>
  Action.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}),
    _: () => player,
  }, action);

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[1])
  }, 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[1])
});

caseOn is similar to case, but allows passing additional data directly into each case function. With caseOn, the advancePlayer example from before could be written in "point-free style" like this:

// No need to wrap this into a function that passes `player`
const advancePlayer = Action.caseOn({
  Up: (player) => ({x: player.x, y: player.y - 1}),
  Right: (player) => ({x: player.x + 1, y: player.y}),
  Down: (player) => ({x: player.x, y: player.y + 1}),
  Left: (player) => ({x: player.x - 1, y: player.y}),
  _: (player) => player
});

advancePlayer(Action.Up, player);

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. The fields will NOT be extracted when matching on _ as this may result in inconsistent argument positions.

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

In addition to the static case and caseOn functions on a type, instances of a type have case and caseOf methods, so for example

Action.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}),
  _: () => player,
}, action);

could equivalently be written as

action.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}),
  _: () => player,
});

Extracting fields from a union type

If your type was defined using the record syntax you can access the fields through the name you specified:

var Person = Type({Person: {name: String, age: Number, shape: Shape}});
var person = Person.PersonOf({name: 'Simon', age: 21, shape: Circle});
var name = person.name;
var age = person.age;
var favoriteShape = person.shape;

If your type was not created using the record syntax the fields have to be extracted by indexing your union type:

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'

Disabling type checking

Type checking can be disabled, for instance in production, by setting Type.check to false.

Author & license

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