newtype-ts

What is newtype-ts?

The newtype-ts package provides utilities for creating and working with newtypes in TypeScript. Newtypes are a way to create distinct types from existing ones without runtime overhead, which can help with type safety and code clarity.

What are newtype-ts's main functionalities?

Creating Newtypes

This feature allows you to create newtypes, which are distinct types derived from existing ones. The example demonstrates creating a newtype for meters and using an isomorphism to wrap and unwrap values.

import { Newtype, iso } from 'newtype-ts';

type Meter = Newtype<{ readonly Meter: unique symbol }, number>;
const meterIso = iso<Meter>();

const distance: Meter = meterIso.wrap(5);
const value: number = meterIso.unwrap(distance);

Using Newtypes with Functions

This feature shows how to use newtypes with functions to ensure type safety. The example defines newtypes for meters and seconds, and a function to calculate speed, ensuring that only the correct types are used.

import { Newtype, iso } from 'newtype-ts';

type Meter = Newtype<{ readonly Meter: unique symbol }, number>;
const meterIso = iso<Meter>();

type Second = Newtype<{ readonly Second: unique symbol }, number>;
const secondIso = iso<Second>();

const speed = (distance: Meter, time: Second): number => meterIso.unwrap(distance) / secondIso.unwrap(time);

const distance: Meter = meterIso.wrap(100);
const time: Second = secondIso.wrap(10);

const result = speed(distance, time); // 10

Combining Newtypes

This feature demonstrates how to combine newtypes to create more complex types. The example shows how to calculate speed from distance and time, and wrap the result in a newtype for speed.

import { Newtype, iso } from 'newtype-ts';

type Meter = Newtype<{ readonly Meter: unique symbol }, number>;
const meterIso = iso<Meter>();

type Second = Newtype<{ readonly Second: unique symbol }, number>;
const secondIso = iso<Second>();

type Speed = Newtype<{ readonly Speed: unique symbol }, number>;
const speedIso = iso<Speed>();

const calculateSpeed = (distance: Meter, time: Second): Speed => speedIso.wrap(meterIso.unwrap(distance) / secondIso.unwrap(time));

const distance: Meter = meterIso.wrap(100);
const time: Second = secondIso.wrap(10);

const speed: Speed = calculateSpeed(distance, time);

Other packages similar to newtype-ts

io-ts

io-ts is a runtime type system for IO decoding/encoding in TypeScript. It provides a way to define types that can be checked at runtime, which is useful for validating external data. Unlike newtype-ts, io-ts focuses on runtime type validation rather than creating distinct types at compile time.

runtypes

runtypes is a library for runtime validation and type checking in TypeScript. It allows you to define types that can be validated at runtime, similar to io-ts. While runtypes provides runtime type safety, newtype-ts focuses on creating distinct types at compile time without runtime overhead.

typescript-is

typescript-is is a TypeScript transformer that allows you to perform type checking at runtime. It provides a way to assert types and validate data at runtime. Unlike newtype-ts, which focuses on compile-time type safety, typescript-is provides runtime type assertions.

README

Installation

To install the stable version:

npm i newtype-ts

Note. newtype-ts depends on fp-ts and monocle-ts, starting from 0.3.0 you must install both fp-ts and monocle-ts manually (fp-ts and monocle-ts are listed in peerDependency)

Motivation

A common programming practice is to define a type whose representation is identical to an existing one but which has a separate identity in the type system.

type USD = number
type EUR = number

const myamount: USD = 1

declare function change(usd: USD): EUR
declare function saveAmount(eur: EUR): void

saveAmount(change(myamount)) // ok
saveAmount(myamount) // opss... this is also ok because both EUR and USD are type alias of number!

Usage

Newtypes

Let's define a newtype for the EUR currency

import { Newtype, iso } from 'newtype-ts'

interface EUR extends Newtype<{ readonly EUR: unique symbol }, number> {}

// isoEUR: Iso<EUR, number>
const isoEUR = iso<EUR>()

// myamount: EUR
const myamount = isoEUR.wrap(0.85)

// n: number = 0.85
const n = isoEUR.unwrap(myamount)

declare function saveAmount(eur: EUR): void

saveAmount(0.85) // static error: Argument of type '0.85' is not assignable to parameter of type 'EUR'
saveAmount(myamount) // ok

By definition a "newtype" must have the exact same runtime representation as the value that it stores, e.g. a value of type EUR is just a number at runtime.

For the Iso type, see the monocle-ts documentation.

Refinements

An Integer is a refinement of number

import { Newtype, prism } from 'newtype-ts'

interface Integer extends Newtype<{ readonly Integer: unique symbol }, number> {}

const isInteger = (n: number) => Number.isInteger(n)

// prismInteger: Prism<number, Integer>
const prismInteger = prism<Integer>(isInteger)

// oi: Option<Integer>
const oi = prismInteger.getOption(2)

declare function f(i: Integer): void

f(2) // static error: Argument of type '2' is not assignable to parameter of type 'Integer'
oi.map(f) // ok

For the Prism type, see the monocle-ts documentation.

Builtin refinements

• Char
• Integer
• Negative
• NegativeInteger
• NonNegative
• NonNegativeInteger
• NonPositive
• NonPositiveInteger
• NonEmptyString
• NonZero
• NonZeroInteger
• Positive
• PositiveInteger

import { NonZero, prismNonZero } from 'newtype-ts/lib/NonZero'

// a total function
const safeDivide = (numerator: number, denominator: NonZero): number => {
  return numerator / prismNonZero.reverseGet(denominator)
}

// result: Option<number>
const result = prismNonZero.getOption(2).map(denominator => safeDivide(2, denominator))

TypeScript compatibility

The stable version is tested against TypeScript 3.5.1

newtype-ts version	required typescript version	required fp-ts version	required monocle-ts version
0.3	3.5.1+	2.0.0-rc.6+	2.0.0-rc.1+
<= 0.2.4	2.8+	1.0.0+	1.0.0+

Performance

const double = n => n * 2
const doubleEUR = eurIso.modify(double)

Test double(2) vs doubleEUR(eurIso.wrap(2))

Results (node v8.9.3)

double x 538,301,203 ops/sec ±0.45% (87 runs sampled)
doubleEUR x 536,575,600 ops/sec ±0.27% (87 runs sampled)

Recipes

How to lift a function

const double = (n: number): number => n * 2

// doubleEUR: (s: EUR) => EUR
const doubleEUR = eurIso.modify(double)

Documentation

• API Reference