"Si vis pacem, para bellum" (Vegetius 5th century)

tcomb is a library for Node.js and the browser which allows you to check the types of JavaScript values at runtime with a simple syntax. It's great for Domain Driven Design, for testing and for adding safety to your internal code.

Contributors

• Giulio Canti
• Becky Conning 'func' combinator ideas and documentation.

Contents

• Features
• Quick Examples
• Setup
• Requirements
• Tests
• The Idea
• Api
  • options
    • options.onFail
    • options.update
  • assert
  • structs
  • unions
  • maybe
  • enums
  • subtypes
  • lists
  • dicts
  • functions
• sweet.js macros (experimental)
• Articles on tcomb

Features

• write complex domain models in a breeze and with small code footprint
• easy debugging
• instances are immutables by default
• encode/decode of domain models to/from JSON for free

The library provides a built-in assert function, if an assert fails the debugger kicks in so you can inspect the stack and quickly find out what's wrong.

You can handle:

JavaScript native types

• Nil: null and undefined
• Str: strings
• Num: numbers
• Bool: booleans
• Arr: arrays
• Obj: plain objects
• Func: functions
• Err: errors
• Re: regular expressions
• Dat: dates
• Any: *

type combinators (build new types from those already defined)

• struct (i.e. classes)
• union
• maybe
• enums
• tuple
• subtype
• list
• dict
• function type

Quick Examples

Let's build a product model

var Product = struct({
    name: Str,                  // required string
    desc: maybe(Str),           // optional string, can be null
    home: Url,                  // a subtype of a string
    shippings: list(Str),       // a list of shipping methods
    category: Category,         // enum, one of [audio, video]
    price: Price,               // a price (dollars) OR in another currency
    size: tuple([Num, Num]),    // width x height
    warranty: dict(Str, Num)    // a dictionary country -> covered years
});

var Url = subtype(Str, function (s) {
    return s.indexOf('http://') === 0;
});
var Category = enums.of('audio video');
var ForeignPrice = struct({ currency: Str, amount: Num });
var Price = union([Num, ForeignPrice]);
Price.dispatch = function (x) {
  return Num.is(x) ? Num : ForeignPrice;
};

// JSON of a product
var json = {
    name: 'iPod',
    desc: 'Engineered for maximum funness.',
    home: 'http://www.apple.com/ipod/',
    shippings: ['Same Day', 'Next Businness Day'],
    category: 'audio',
    price: {currency: 'EUR', amount: 100},
    size: [2.4, 4.1],
    warranty: {
      US: 2,
      IT: 1
    }
};

// get an immutable instance, `new` is optional
var ipod = Product(json);

You have existing code and you want to add safety

// your code: plain old JavaScript class
function Point (x, y) {
    this.x = x;
    this.y = y;
}

var p = new Point(1, 'a'); // silent error

in order to "tcombify" your code you can simply add some asserts

function Point (x, y) {
    assert(Num.is(x));
    assert(Num.is(y));
    this.x = x;
    this.y = y;
}

var p = new Point(1, 'a'); // => fail! debugger kicks in

Setup

Node

npm install tcomb

Browser

bower install tcomb

or download the tcomb.min.js file.

Requirements

This library uses a few ES5 methods, you can use es5-shim, es5-sham and json2 to support old browsers

Tests

Run mocha or npm test in the project root.

The Idea

What's a type? In tcomb a type is a function T such that

1. T has signature T(value, [mut]) where value depends on the nature of T and the optional boolean mut makes the instance mutable (default false)
2. T is idempotent: T(T(value, mut), mut) === T(value, mut)
3. T owns a static function T.is(x) returning true if x is an instance of T

Note: 2. implies that T can be used as a default JSON decoder

Articles on tcomb

• JavaScript, Types and Sets Part 1
• JavaScript, Types and Sets Part 2
• What if your domain model could validate the UI for free?
• JSON Deserialization Into An Object Model
• JSON API Validation In Node.js

Api

options

options.onFail

In production envs you don't want to leak failures to the user

// override onFail hook
options.onFail = function (message) {
    try {
        // capture stack trace
        throw new Error(message);
    } catch (e) {
        // use you favourite JavaScript error logging service
        console.log(e.stack);
    }
};

options.update

Adds to structs, tuples, lists and dicts a static method update that returns a new instance without modifying the original.

Example

// see http://facebook.github.io/react/docs/update.html
options.update = function (x, updates) {
  return React.addons.update(mixin({}, x), updates);
};
var p1  = Point({x: 0, y: 0});
var p2 = Point.update(p1, {x: {$set: 1}}); // => Point({x: 1, y: 0})

assert

assert(guard, [message], [values...]);

If guard !== true the debugger kicks in.

• guard boolean condition
• message optional string useful for debugging, formatted with values like util.format in Node

Example

assert(1 === 2); // throws 'assert(): failed'
assert(1 === 2, 'error!'); // throws 'error!'
assert(1 === 2, 'error: %s !== %s', 1, 2); // throws 'error: 1 !== 2'

To customize failure behaviour, see options.onFail.

structs

struct(props, [name])

Defines a struct like type.

• props hash name -> type
• name optional string useful for debugging

Example

"use strict";
  
// defines a struct with two numerical props
var Point = struct({
    x: Num,
    y: Num
});
  
// methods are defined as usual
Point.prototype.toString = function () {
    return '(' + this.x + ', ' + this.y + ')';
};
  
// costructor usage, p is immutable
var p = Point({x: 1, y: 2});
  
p.x = 2; // => TypeError
  
p = Point({x: 1, y: 2}, true); // now p is mutable
  
p.x = 2; // ok

is(x)

Returns true if x is an instance of the struct.

Point.is(p); // => true

extend(props, [name])

Returns a new type with the additional specified props

var Point = struct({
  x: Num,
  y: Num
}, 'Point');

var Point3D = Point.extend({z: Num}, 'Point3D'); // composition, not inheritance

var p = new Point3D({x: 1, y: 2, z: 3});

unions

union(types, [name])

Defines a union of types.

• types array of types
• name optional string useful for debugging

Example

var Circle = struct({
    center: Point,
    radius: Num
});
  
var Rectangle = struct({
    bl: Point, // bottom left vertex
    tr: Point  // top right vertex
});
  
var Shape = union([
    Circle, 
    Rectangle
]);

// you must implement the dispatch() function in order to use `Shape` as a contructor
Shape.dispatch = function (x) {
  return x.hasOwnProperty('center') ? Circle : Rectangle;
};

var circle = Shape({center: {x: 1, y: 0}, radius: 10});
var rectangle = Shape({bl: {x: 0, y: 0}, tr: {x: 1, y: 1}});

is(x)

Returns true if x belongs to the union.

Shape.is(new Circle({center: p, radius: 10})); // => true

maybe

maybe(type, [name])

Same as union([Nil, type]).

// the value of a radio input where null = no selection
var Radio = maybe(Str);
  
Radio.is('a');     // => true
Radio.is(null);    // => true
Radio.is(1);       // => false

enums

enums(map, [name])

Defines an enum of strings.

• map hash enum -> value
• name optional string useful for debugging

Example

var Direction = enums({
    North: 'North', 
    East: 'East',
    South: 'South', 
    West: 'West'
});

is(x)

Returns true if x belongs to the enum.

Direction.is('North'); // => true

enums.of(keys, [name])

Returns an enums of an array of keys, useful when you don't mind to define custom values for the enums.

• keys array (or string) of keys
• name optional string useful for debugging

Example

// result is the same as the main example
var Direction = enums.of(['North', 'East', 'South', 'West']);
  
// or..
Direction = enums.of('North East South West');

tuples

tuple(types, [name])

Defines a tuple whose coordinates have the specified types.

• types array of coordinates types
• name optional string useful for debugging

Example

var Area = tuple([Num, Num]);

// constructor usage, area is immutable
var area = Area([1, 2]);

0-tuples and 1-tuples are also supported

var Nothing = tuple([]);
var JustNum = tuple([Num]);

is(x)

Returns true if x belongs to the tuple.

Area.is([1, 2]);      // => true
Area.is([1, 'a']);    // => false, the second element is not a Num
Area.is([1, 2, 3]);   // => false, too many elements

subtypes

subtype(type, predicate, [name])

Defines a subtype of an existing type.

• type the supertype
• predicate a function with signature (x) -> boolean
• name optional string useful for debugging

Example

// points of the first quadrant
var Q1Point = subtype(Point, function (p) {
    return p.x >= 0 && p.y >= 0;
});
  
// costructor usage, p is immutable
var p = Q1Point({x: 1, y: 2});
  
p = Q1Point({x: -1, y: -2}); // => fail!

Note. You can't add methods to Q1Point prototype, add them to the supertype prototype if needed.

is(x)

Returns true if x belongs to the subtype.

var Int = subtype(Num, function (n) {
    return n === parseInt(n, 10);
});
  
Int.is(2);      // => true
Int.is(1.1);    // => false

lists

list(type, [name])

Defines an array where all the elements are of type T.

• type type of all the elements
• name optional string useful for debugging

Example

var Path = list(Point);
  
// costructor usage, path is immutable
var path = Path([
    {x: 0, y: 0}, 
    {x: 1, y: 1}
]);

is(x)

Returns true if x belongs to the list.

var p1 = Point({x: 0, y: 0});
var p2 = Point({x: 1, y: 2});
Path.is([p1, p2]); // => true

dicts

dict(domain, codomain, [name])

Defines a dictionary domain -> codomain.

• domain the type of the keys
• codomain the type of the values
• name optional string useful for debugging

Example

// defines a dictionary of numbers
var Tel = dict(Str, Num);

is(x)

Returns true if x is an instance of the dict.

Tel.is({'jack': 4098, 'sape': 4139}); // => true

functions

Typed functions may be defined like this:

// add takes two `Num`s and returns a `Num`
var add = func([Num, Num], Num)
    .of(function (x, y) { return x + y; });

And used like this:

add("Hello", 2); // Raises error: Invalid `Hello` supplied to `Num`
add("Hello");    // Raises error: Invalid `Hello` supplied to `Num`

add(1, 2);       // Returns: 3
add(1)(2);       // Returns: 3

func(A, B, [name])

func(Domain, Codomain, name)

Returns a function type whose functions have their domain and codomain specified and constrained.

• Domain: the type of the function's argument (or list of types of the function's arguments)
• Codomain: the type of the function's return value
• name: optional string useful for debugging

func can be used to define function types using native types:

// An `A` takes a `Str` and returns an `Num`
var A = func(Str, Num);

The domain and codomain can also be specified using types from any combinator including func:

// A `B` takes a `Func` (which takes a `Str` and returns a `Num`) and returns a `Str`.
var B = func(func(Str, Num), Str);

// An `ExcitedStr` is a `Str` containing an exclamation mark
var ExcitedStr = subtype(Str, function (s) { return s.indexOf('!') !== -1; }, 'ExcitedStr');

// An `Exciter` takes a `Str` and returns an `ExcitedStr`
var Exciter = func(Str, ExcitedStr);

Additionally the domain can be expressed as a list of types:

// A `C` takes an `A`, a `B` and a `Str` and returns a `Num`
var C = func([A, B, Str], Num);

.of(f)

func(A, B).of(f);

Returns a function where the domain and codomain are typechecked against the function type.

If the function is passed values which are outside of the domain or returns values which are outside of the codomain it will raise an error:

var simpleQuestionator = Exciter.of(function (s) { return s + '?'; });
var simpleExciter      = Exciter.of(function (s) { return s + '!'; });

// Raises error: 
// Invalid `Hello?` supplied to `ExcitedStr`, insert a valid value for the subtype
simpleQuestionator('Hello');

// Raises error: Invalid `1` supplied to `Str`
simpleExciter(1);

// Returns: 'Hello!'
simpleExciter('Hello');

The returned function may also be partially applied:

// We can reasonably suggest that add has the following type signature
// add : Num -> Num -> Num
var add = func([Num, Num], Num)
    .of(function (x, y) { return x + y });

var addHello = add("Hello"); // As this raises: "Error: Invalid `Hello` supplied to `Num`"

var add2 = add(2);
add2(1); // And this returns: 3

.is(x)

func(A, B).is(x);

Returns true if x belongs to the type.

Exciter.is(simpleExciter);      // Returns: true
Exciter.is(simpleQuestionator); // Returns: true

var id = function (x) { return x; };

func([Num, Num], Num).is(func([Num, Num], Num).of(id)); // Returns: true
func([Num, Num], Num).is(func(Num, Num).of(id));        // Returns: false 

Rules

1. Typed functions' domains are checked when they are called
2. Typed functions' codomains are checked when they return
3. The domain and codomain of a typed function's type is checked when the typed function is passed to a function type (such as when used as an argument in another typed function).

Macros (experimental)

I added a tcomb.sjs file containing some sweet.js macros, here some examples:

// structs
type Point struct {
  x: Num,
  y: Num
}

// unions
type Shape union {
  Circle,
  Rectangle
}

// tuples
type Coords tuple {
  Num,
  Num
}

// enums
type Direction enums {
  'North', 
  'East',
  'South', 
  'West'
}

// enums with specified values 
type Direction enums {
  'North': 0, 
  'East': 1,
  'South': 2, 
  'West': 3
}

// subtypes
type Positive subtype<Num> n {
  return n > 0;
}

// irriducibles types, like JavaScript primitives
type Irriducible irriducible x {
  return typeof x === 'function';
}

// maybe
type Maybe maybe<Str>

// lists
type List list<Str>

// dictionaries
type Dict dict<Str>

// functions
fn add (x: Num, y: Num) {
    return x + y;
}

// functions with checked return value
fn add (x: Num, y: Num) -> Num {
    return x + y;
}

License

The MIT License (MIT)