Smart Types TS

A collection of Smart Types and Smart Constructors which enable you to be more strict when defining your application's important types/interfaces.

Contents

• Definitions
• Usage examples

Definitions

What is a Smart Type?

A Smart Type is a type which cannot be created without going through a special function.

For example, trying to directly assign a variable the EmailAddress type will not work:

import { EmailAddress } from "smart-types-ts";

const myEmail: EmailAddress = "[email protected]"; // TS Error!

What is a Smart Constructor?

A Smart Constructor is a function which produces a Smart Type. All smart constructors in this library take some input and return either a string or a Smart Type (Either<string, SmartType>).

The Either type comes from the fp-ts library. You can read more about how to work with this here or you can read the documentation.

For example, to create an EmailAddress value, you can use the corresponding mkEmailAddress Smart Constructor:

import { mkEmailAddress } from "smart-types-ts";

mkEmailAddress("[email protected]"); // Right("[email protected]")

mkEmailAddress("bad-input"); // Left("Not a valid email address")

Usage

Define your domain types using the relevant types from smart-types-ts:

import { EmailAddress, StringWithLength, URL } from "smart-types-ts";

interface User {
  email: EmailAddress;
  name: {
    display: StringWithLength<1, 30>;
    full: StringWithLength<1, 100>;
  };
  profilePicture: URL;
}

Define functions to convert simple objects to your smart types:

import {
  mkEmailAddress,
  mkObject,
  mkStringWithLength,
  mkURL,
} from "smart-types-ts";

// Define our mkUser function which can be used to construct a User
const mkUser = mkSmartObject<User>({
  email: mkEmailAddress,
  // use mkSmartObject again for nested objects
  name: mkSmartObject({
    display: mkStringWithLength<1, 30>,
    full: mkStringWithLength<1, 100>
  }),
  profilePicture: mkUrl,
});


mkUser({ email: "bleh", name: { display: "", full: "" }, profilePicture: "bad-url" });
// Left({
//   email: "Not a valid email",
//   name: {
//     display: "Length not between 1-30",
//     full: "Length not between 1-100",
//   },
//   profilePicture: "Not a valid URL"
// })

mkUser({
  email: "[email protected]",
  name: { display: "Jane", full: "Jane Doe" },
  profilePicture: "https://www.example.com/photo/1",
});
// Right(User)

What problem does this library solve?

The Typescript compiler is a powerful tool which developers can leverage to guarantee that their code behaves correctly. Types provide clear definitions of the important entities in a program.

However, the default arbitrary types available in the language are not descriptive enough to limit invalid data within a program.

Let's look at a simple example of a User which has an email, a username and a password. This could be represented by the following interface:

interface User {
  email: string;
  fullName: string;
  profilePicture: string;
}

There are a number of problems with this type. It doesn't tell us anything about what values are valid for each of the fields. An email should only ever contain a valid email address. We may want the fullName to have a minimum length of 1 character and a maximum length of 50 characters. The profilePicture should be a valid URL pointing to the location of the photo.

What if instead we were able to define these constrains in terms of types?

This could look like:

interface User {
  email: EmailAddress;
  fullName: StringOfLength<1, 50>;
  profilePicture: URL;
}

smart-types-ts simply provides a large number of these Smart Types and their corresponding constructors (called Smart Constructors!) ready for you to use, so that you can get on with modelling your application's domain without having to deal with complex Typescript tricks.

It also provides the mkSmartObject function which allows you to build your smart objects from unknown inputs. This is useful for validating/parsing data from HTTP requests, files, databases, or any external input to your program.