The idea

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

has signature T(value) where value depends on the nature of T
is idempotent, that is T(T(value)) = T(value)
owns a static function T.is(x) returning true if x is an instance of T

The `meta` object

Every type defined with tcomb owns a static meta member containing at least the following properties:

kind an enum containing the type kind ('irreducible', 'struct', etc...)
name an optional string, useful for debugging purposes
identity a boolean, true if the type can be treated as the identity function in production builds

See the documentation of each combinator for specific additional properties.

Note. Meta objects are a distinctive feature of tcomb, allowing runtime type introspection.

The `assert` function

Signature

(guard: boolean, message?: string | () => string) => void

Example

const x = 0;
t.assert(x !== 0, 'cannot divide by x'); // => throws '[tcomb] cannot divide by x'

The `fail` function

Signature

(message: string) => void

Note. You can change the default behaviour when an assert fails overriding the t.fail function.

The built-in types

t.String: strings
t.Number: numbers
t.Integer: integers
t.Boolean: booleans
t.Array: arrays
t.Object: plain objects
t.Function: functions
t.Error: errors
t.RegExp: regular expressions
t.Date: dates
t.Nil: null or undefined
t.Any: any value
t.Type: a tcomb type

The `irreducible` combinator

Signature

(name: string, predicate: (x: any) => boolean) => TcombType

Example. Representing a native Promise:

const PromiseType = t.irreducible('PromiseType', (x) => x instanceof Promise);

The meta object

{
  kind: 'irreducible',
  name: name,
  identity: true,
  predicate: predicate
}

Note. All the built-in types exported by tcomb are defined through the irreducible combinator.

The `refinement` combinator

Signature

(type: tcombType, predicate: (x: any) => boolean, name?: string) => TcombType

Example. Representing a positive number:

const Positive = t.refinement(t.Number, (n) => n >= 0, 'Positive');

The meta object

{
  kind: 'subtype',
  name: name,
  identity: ...depends on type,
  type: type,
  predicate: predicate
}

The `enums` combinator

Signature

(map: Object, name?: string) => TcombType

where map is a hash whose keys are the enums (values are free).

Example

const Country = t.enums({
  IT: 'Italy',
  US: 'United States'
}, 'Country');

The meta object

{
  kind: 'enums',
  name: name,
  identity: true,
  map: map
}

The `of` static function

If you don't care of values you can use enums.of:

(keys: string | Array<string | number>, name?: string) => TcombType

where keys is the array of enums or a string where the enums are separated by spaces.

Example

// values will mirror the keys
const Country = t.enums.of('IT US', 'Country');

// same as
const Country = t.enums.of(['IT', 'US'], 'Country');

// same as
const Country = t.enums({
  IT: 'IT',
  US: 'US'
}, 'Country');

The `maybe` combinator

In tcomb optional values of type T can be represented by union([Nil, T]). Since it's very common to handle optional values, tcomb provide an ad-hoc combinator.

Signature

(type: tcombType, name?: string) => TcombType

Example

const Person = t.struct({
  name: t.String,
  age: t.maybe(t.Number) // an optional number
});

Person({ name: 'Giulio' }); // => ok
Person({ name: 'Giulio', age: null }); // => ok
Person({ name: 'Giulio', age: undefined }); // => ok
Person({ name: 'Giulio', age: 'a string' }); // => throws

The meta object

{
  kind: 'maybe',
  name: name,
  identity: ...depends on type,
  type: type
}

The `struct` combinator

Signature

type Options = {
  name?: string,
  strict?: boolean,
  defaultProps?: Object
};

(props: {[key: string]: TcombType;}, options?: string | Options) => TcombType

Example

const Point = t.struct({
  x: t.Number,
  y: t.Number
}, 'Point');

// point is immutable, the new keyword is optional
const point = Point({ x: 1, y: 2 });

Note. Point.is internally uses instanceof.

The meta object

{
  kind: 'struct',
  name: options.name,
  identity: false,
  props: props,
  strict: options.strict,
  defaultProps: options.defaultProps
}

Strictness

If strict = true then no additional props are allowed:

const Point = t.struct({
  x: t.Number,
  y: t.Number
}, 'Point');

Point({ x: 1, y: 2, z: 3 }); // => ok

const Point = t.struct({
  x: t.Number,
  y: t.Number
}, { name: 'Point', strict: true });

Point({ x: 1, y: 2, z: 3 }); // => throws '[tcomb] Invalid additional prop "z" supplied to Point'

Defining methods on a struct

Methods are defined as usual:

Point.prototype.toString = function () {
  return `(${this.x}, ${this.y})`;
};

Extending structs

Every struct constructor owns an extend function:

type Props = {[key: String]: Type};
type Mixin = Props | TcombStruct | TcombInterface | refinement(Mixin);
type Options = {
  name?: string,
  strict?: boolean,
  defaultProps?: Object
};

extend(mixins: Mixin | Array<Mixin>, options?: : string | Options) => TcombStruct

Example

const Point3D = Point.extend({ z: t.Number }, 'Point3D');

// multiple inheritance
const A = struct({...});
const B = struct({...});
const MixinC = {...};
const MixinD = {...};
const E = A.extend([B, MixinC, MixinD]);

Note. extend supports prototypal inheritance:

const Rectangle = t.struct({
  width: t.Number,
  height: t.Number
});

Rectangle.prototype.getArea = function () {
  return this.width * this.height;
};

const Cube = Rectangle.extend({
  thickness: t.Number
});

// typeof Cube.prototype.getArea === 'function'
Cube.prototype.getVolume = function () {
  return this.getArea() * this.thickness;
};

Note. Repeated props are not allowed (unless they are strictly equal):

const Wrong = Point.extend({ x: t.String }); // => throws '[tcomb] Invalid call to mixin(target, source, [overwrite]): cannot overwrite property "x" of target object'

Alternatively you can use the t.struct.extend(mixins: Array<Mixin>, options?: string | Options) => TcombType function:

const Point3D = t.struct.extend([Point, { z: t.Number }], 'Point3D');

Note. The implementation uses the top level function extend(combinator, mixins, options) defined in tcomb/lib/extend

The `tuple` combinator

Signature

(types: Array<TcombType>, name?: string) => TcombType

Note. Instances of tuples are plain old JavaScript arrays.

Example

const Area = t.tuple([t.Number, t.Number]);

// area is immutable
const area = Area([1, 2]);

The meta object

{
  kind: 'tuple',
  name: name,
  identity: ...depends on types,
  types: types
}

The `list` combinator

Signature

(type: TcombType, name?: string) => TcombType

Note. Instances of lists are plain old JavaScript arrays.

Example

const Path = t.list(Point);

// path is immutable
const path = Path([
  {x: 0, y: 0}, // tcomb automatically hydrates using the `Point` constructor
  {x: 1, y: 1}
]);

The meta object

{
  kind: 'list',
  name: name,
  identity: ...depends on type,
  type: type
}

The `dict` combinator

Signature

(domain: TcombType, codomain: TcombType, name?: string) => TcombType

Note. Instances of dicts are plain old JavaScript objects.

Example

const Phones = t.dict(t.String, t.Number);

// phones is immutable
const phones = Tel({ 'jack': 4098, 'sape': 4139 });

The meta object

{
  kind: 'dict',
  name: name,
  identity: ..depends on domain and codomain,
  domain: domain,
  codomain: codomain
}

The `union` combinator

Signature

(types: Array<TcombType>, name?: string) => TcombType

Example

const IncrementAction = t.struct({ step: t.Number });
const DecrementAction = t.struct({ step: t.Number });
const Actions = t.union([IncrementAction, DecrementAction])

The meta object

{
  kind: 'union',
  name: name,
  identity: ...depends on types,
  types: types
}

The `dispatch` function

In order to use a union as a constructor you must implement the dispatch static function:

(x: any) => TcombType

Example

Actions.dispatch = function dispatch(x) {
  const typeToConstructor = {
    'increment': IncrementAction,
    'decrement': DecrementAction
  };
  return typeToConstructor[x.type];
};

const incrementAction = Action({ type: 'increment', step: 1 });

Note. tcomb provides a default implementation of dispatch which you can override.

The `intersection` combinator

Signature

(types: Array<TcombType>, name?: string) => TcombType

Example

const Min = t.refinement(t.String, (s) => s.length > 2);
const Max = t.refinement(t.String, (s) => s.length < 5);
const MinMax = t.intersection([Min, Max]);

MinMax.is('abc');   // => true
MinMax.is('a');     // => false
MinMax.is('abcde'); // => false

The meta object

{
  kind: 'intersection',
  name: name,
  identity: ...depends on types,
  types: types
}

The `interface` combinator

There is an alias (inter) for IE8 compatibility.

Differences from structs

is doesn't leverage instanceof, structural typing is used instead
doesn't filter additional props
also checks prototype keys

Signature

type Options = {
  name?: string,
  strict?: boolean
};

(props: {[key: string]: TcombType;}, options?: string | Options) => TcombType

Example

const Foo = t.interface({
  x: t.Number,
  y: t.Number
}, 'Foo');

var foo = Foo({ x: 1, y: 2 }); // => { x: 1, y: 2 }
foo instanceof Foo; // => false (it's a pojo)
Foo.is({ x: 1, y: 2 }); // => true

// allows additional props
Foo.is({ x: 1, y: 2, z: 3 }); // => true

// checks types
Foo.is({ x: 1, y: '2' }); // => false

// doesn't allow missing props
Foo.is({ x: 1 }); // => false

const Point = t.struct({
  x: t.Number,
  y: t.Number
}, 'Point');

const Bar = t.interface({
  point: Point
}, 'Bar');

// hydrates prop values
const bar = Bar({ point: {x: 0, y: 0} });
Point.is(bar.point); // => true

const Serializable = t.interface({
  serialize: t.Function
})

Point.prototype.serialize = function () {
  ...
};

function doSerialize(serializable: Serializable) {
  ...
}

doSerialize(bar.point); // => ok

The meta object

{
  kind: 'interface',
  name: options.name,
  identity: ...depends on props,
  props: props,
  strict: options.strict
}

Strictness

If strict = true then no additional props or prototype keys are allowed:

Foo({ x: 1, y: 2, z: 3 }); // => ok

const Foo = t.interface({
  x: t.Number,
  y: t.Number
}, { name: 'Foo', strict: true });

Foo({ x: 1, y: 2, z: 3 }); // => throws '[tcomb] Invalid additional prop "z" supplied to Foo'

Foo(new Point({ x: 1, y: 2 })); // => throws '[tcomb] Invalid additional prop "serialize" supplied to Foo'

Extending interfaces

Every interface constructor owns an extend function:

type Props = {[key: String]: Type};
type Mixin = Props | TcombStruct | TcombInterface | refinement(Mixin);
type Options = {
  name?: string,
  strict?: boolean
};

extend(mixins: Mixin | Array<Mixin>, options?: string | Options) => TcombStruct

Example

const Point3D = Point.extend({ z: t.Number }, 'Point3D');

// multiple inheritance
const A = interface({...});
const B = struct({...});
const MixinC = {...};
const MixinD = {...};
const E = A.extend([B, MixinC, MixinD]);

Note. Repeated props are not allowed (unless they are strictly equal):

const Wrong = Point.extend({ x: t.String }); // => throws '[tcomb] Invalid call to mixin(target, source, [overwrite]): cannot overwrite property "x" of target object'

Alternatively you can use the t.interface.extend(mixins: Array<Mixin>, options?: string | Options) => TcombType function:

const Point3D = t.interface.extend([Point, { z: t.Number }], 'Point3D');

Note. The implementation uses the top level function extend(combinator, mixins, options) defined in tcomb/lib/extend

The `func` combinator

Signature

(domain: Array<TcombType>, codomain: TcombType, name?: string) => TcombType

Example

Typed functions may be defined like this:

// add takes two `t.Number`s and returns a `t.Number`
const add = t.func([t.Number, t.Number], t.Number)
  .of((x, y) => x + y);

And used like this:

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

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

You can define a typed function using the func(domain, codomain, name?) combinator where:

domain is the type of the function's argument (or list of types of the function's arguments)
codomain is the type of the function's return value
name: is an optional string useful for debugging purposes

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

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

// An `A` takes a `t.String` and returns an `t.Number`
const A = t.func(t.String, t.Number);

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

// A `B` takes a `Func` (which takes a `t.String` and returns a `t.Number`) and returns a `t.String`.
const B = t.func(t.func(t.String, t.Number), t.String);

// An `ExcitedString` is a `t.String` containing an exclamation mark
const ExcitedString = t.refinement(t.String, (s) => s.indexOf('!') !== -1, 'ExcitedString');

// An `Exciter` takes a `t.String` and returns an `ExcitedString`
const Exciter = t.func(t.String, ExcitedString);

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

// A `C` takes an `A`, a `B` and a `t.String` and returns a `t.Number`
const C = t.func([A, B, t.String], t.Number);

The meta object

{
  kind: 'func',
  name: name,
  identity: true,
  domain: domain,
  codomain: codomain
}

The `of(f: Function, curried?: boolean)` function

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:

const simpleQuestionator = Exciter.of((s) => s + '?');
const simpleExciter      = Exciter.of((s) => s + '!');

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

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

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

The returned function may also be partially applied passing a curried additional param:

// We can reasonably suggest that add has the following type signature
// add : t.Number -> t.Number -> t.Number
const add = t.func([t.Number, t.Number], t.Number)
  .of((x, y) => x + y, true);

const addHello = add("Hello"); // As this raises: "Error: Invalid `Hello` supplied to `t.Number`"

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

The `is(x: any)` function

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

Returns true if x belongs to the type.

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

const id = (x) => x;

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

Rules

Typed functions' domains are checked when they are called
Typed functions' codomains are checked when they return
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)

Recursive types

t.declare([name]) declares a type name to be used in other combinators without requiring a definition right away. This enables the construction of recursive or mutually recursive types.

const Tree = t.declare('Tree');

Tree.define(t.struct({
  value: t.Number,
  left: t.maybe(Tree),
  right: t.maybe(Tree)
}));

const bst = Tree({
  value: 5,
  left: {
    value: 2
  },
  right: {
    left: {
      value: 6
    },
    value: 7
  }
});

const A = t.declare('A');

const B = t.struct({
  a: t.maybe(A)
});

A.define(t.struct({
  b: t.maybe(B)
}));

Warning. Do not try to type-check structures with circular references, that would blow the stack.

Warning. If you define a union with a custom dispatch, do define the dispatch function before calling define

const U = t.declare('U')
// good
U.dispatch = (x) => t.String.is(x) ? t.String : U
U.define(t.union([t.String, t.list(U)]))
// bad
// U.dispatch = (x) => t.String.is(x) ? t.String : U

Updating immutable instances

You can update an immutable instance with the provided update function:

MyTcombType.update(instance, patch)

The following commands are compatible with the Facebook Immutability Helpers:

$push
$unshift
$splice
$set
$apply
$merge

Example:

const p = Point({ x: 1, y: 2 });

p = Point.update(p, {x: { '$set': 3 }}); // => { x: 3, y: 2 }

Note. $apply can be used only with shallow cloneable values, i.e. Objects, Arrays and primitive values (counterexample: an instance of Date is not shallow cloneable).

Removing a value from a dict

const MyType = dict(t.String, t.Number);
const instance = MyType({ a: 1, b: 2 });
const updated = MyType.update(instance, { $remove: ['a'] }); // => { b: 2 }

Swapping two list elements

const MyType = list(t.Number);
const instance = MyType([1, 2, 3, 4]);
const updated = MyType.update(instance, { '$swap': { from: 1, to: 2 } }); // => [1, 3, 2, 4]

Adding other commands

You can add your custom commands updating the t.update.commands hash.

Pattern matching, the `match` function

Signature

(x: any, ...cases: Array<Case>) => any

where each Case has the following structure:

type, [guard], block

type a tcomb type
guard an optional predicate (x: any) => any
block a function (x: any) => any called when the match succeeded

Example

const A = t.struct({...});

const result = t.match(1,
  t.String, (s) => 'a string',
  t.Number, (n) => n > 2, (n) => 'a number gt 2', // case with a guard (optional)
  t.Number, (n) => 'a number lte 2',
  A,        (a) => 'an instance of A',
  t.Any,    (x) => 'other...' // catch all
);

console.log(result); // => 'a number lte 2'

Note. If a match is not found an error will be raised.

Other exported functions

The `update` function

Immutability helper for POJOs.

Signature

update(instance: Object, patch: Object) => Object

Example

const x = { a: 1 };
t.update(x, { a: { $set: 2 } }); // => { a: 2 }, x is untouched

Note. You can change the default behaviour overriding the t.update function:

t.update = function (instance, patch) {
  // your implementation here
};

The `getTypeName` function

Returns the name of a tcomb type.

Signature

(type: TcombType) => string

Example

t.getTypeName(t.String); // => 'String'

If a name is not specified when defining the type, a default name will be provided according to http://flowtype.org syntax for type annotations.

t.getTypeName(t.maybe(t.String)); // => ?String
t.getTypeName(t.struct({ name: t.String, surname: t.String })); // => '{name: String, surname: String}'
t.getTypeName(t.union([t.String, t.Number])); // => String | Number
t.getTypeName(t.dict(t.String, t.Number)); // => {[key: String]: Number}
...

The `mixin` function

Safe version of mixin, properties cannot be overwritten.

Signature

(target: Object, source: Object, unsafe?: boolean = false) => Object

Example

t.mixin({ a: 1 }, { b: 2 }); // => { a: 1, b: 2 }
t.mixin({ a: 1 }, { a: 2 }); // => throws
t.mixin({ a: 1 }, { a: 1 }); // => ok
t.mixin({ a: 1 }, { a: 2 }, true); // { a: 2 }

The `stringify` function

Used internally to format the error messages.

Signature

(x: any) => String

Since by default JSON.stringify is used, in a performance intensive application you may want to override it:

// override with a less verbose but much faster function
t.stringify = (x) => x.toString();

The `isType` function

Returns true if x is a tcomb type.

Signature

(x: any) => boolean

The `is` function

Returns true if x is an instance of type.

Signature

(x: any, type: TcombType) => boolean

Other modules

The `lib/fromJSON` module

Generic deserialization function.

Signature

(value: any, type: TcombType) => type

Example

import fromJSON from 'tcomb/lib/fromJSON'

const Person = t.struct({
  name: t.String,
  birthDate: Date
});

const source = {
  name: 'Giulio',
  birthDate: new Date(1973, 10, 30)
};

const json = JSON.parse(JSON.stringify(source));

Person(json); // => throws '[tcomb] Invalid value "1973-11-29T23:00:00.000Z" supplied to Person/birthDate: Date'

const person = fromJSON(json, Person);

assert.ok(person instanceof Person); // => true
assert.deepEqual(person, source); // => ok

You can add a static fromJSON: (jsonValue: any) => any function to your types as a custom reviver.

The `lib/installTypeFormatter` module

Chrome Dev Tools custom formatter for tcomb types.

Chrome (v47+) has support for custom "formatters". A formatter tells Chrome's Dev Tools how to display values in the Console, Scope list, etc. This means we can display Structs, Lists, Dicts and other types, in a much better way.

Essentially, it turns this:

into:

Setup

import installTypeFormatter from 'tcomb/lib/installTypeFormatter'
installTypeFormatter()

The `lib/isSubsetOf` module

Function for determining whether one type is compatible with another type.

Signature

(subset: Type, superset: Type) => boolean

Example

import t from 'tcomb'
import isSubsetOf from 'tcomb/lib/isSubsetOf';

isSubsetOf(t.Integer, t.Number) // => true

const Point = t.interface({
  x: t.Number,
  y: t.Number
}, 'Point');

const Point3D = t.interface({
  x: t.Number,
  y: t.Number,
  z: t.Number
}, 'Point3D');

isSubsetOf(Point3D, Point) // => true

const StrictPoint = t.interface({
  x: t.Number,
  y: t.Number
}, { name: 'StrictPoint', strict: true });

isSubsetOf(Point3D, StrictPoint) // => false

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The idea

The meta object

The assert function

The fail function

The built-in types

The irreducible combinator

The refinement combinator

The enums combinator

The of static function

The maybe combinator

The struct combinator

Strictness

Defining methods on a struct

Extending structs

The tuple combinator

The list combinator

The dict combinator

The union combinator

The dispatch function

The intersection combinator

The interface combinator

Strictness

Extending interfaces

The func combinator

The of(f: Function, curried?: boolean) function

The is(x: any) function

Rules

Recursive types

Updating immutable instances

Pattern matching, the match function

Other exported functions

The update function

The getTypeName function

The mixin function

The stringify function

The isType function

The is function

Other modules

The lib/fromJSON module

The lib/installTypeFormatter module

The lib/isSubsetOf module

The `meta` object

The `assert` function

The `fail` function

The `irreducible` combinator

The `refinement` combinator

The `enums` combinator

The `of` static function

The `maybe` combinator

The `struct` combinator

The `tuple` combinator

The `list` combinator

The `dict` combinator

The `union` combinator

The `dispatch` function

The `intersection` combinator

The `interface` combinator

The `func` combinator

The `of(f: Function, curried?: boolean)` function

The `is(x: any)` function

Pattern matching, the `match` function

The `update` function

The `getTypeName` function

The `mixin` function

The `stringify` function

The `isType` function

The `is` function

The `lib/fromJSON` module

The `lib/installTypeFormatter` module

The `lib/isSubsetOf` module