Hegel is a static type checker library that helps you identify typing errors as early as possible without actually running your code. Just like TypeScript and Flow, Hegel detects any information about type errors that exist in your code while you’re writing.

Hegel incorporates many of TypeScript and Flow’s design principles, such as having type annotations and type inferences, without introducing a new language feature like TypeScript’s enums. When using Hegel, you’re writing pure JavaScript without even needing to add a comment, as is the case with Flow.

In this guide, we’ll show how Hegel is different from both TypeScript and Flow and walk you through how to get started with Hegel in your next project.

Hegel versus TypeScript

Let’s break down some of the most notable differences between Hegel and TypeScript.

Skip type annotation

Hegel has a powerful type inference system that enables you to write fewer type annotations.

// Hegel
const promisify = fn => arg => Promise.resolve(fn(arg));
const id = promisify(x => x);
// And "upperStr" will be inferred as "Promise<string>"
const upperStr = id("It will be inferred").then(str => str.toUpperCase());

// TypeScript
const promisify = fn => arg => Promise.resolve(fn(arg));
const id = promisify(x => x);
// And "upperStr" will be inferred as "Promise<any>"
const upperStr = id("It will be inferred").then(str => str.toUpperCase());

No unexpected runtime errors

TypeScript doesn’t aim to apply a sound or provably correct type system, meaning it doesn’t guarantee that you won’t have any type errors at runtime. Hegel does the opposite, implementing a strong type system to guarantee that your code is valid.

// Hegel
const doubles: Array<number> = [Math.PI, Math.E];
// Error: Type "Array<number>" is incompatible with type "Array<number | string>"
const numbersToShow: Array<number | string> = doubles;
numbersToShow.push(42..toString(2));
const rounded = doubles.map(double => double.toFixed());

// TypeScript
const doubles: Array<number> = [Math.PI, Math.E];
const numbersToShow: Array<number | string> = doubles;
numbersToShow.push(42..toString(2));
// Uncaught TypeError: double.toFixed is not a function
doubles.map(double => double.toFixed());

Typed errors

Hegel implemented inference and annotation for functions, which enables you to understand what error is being thrown by the code.

// Type of "assertIsTrue" function is "(boolean) => undefined throws TypeError"
function assertIsTrue(arg) {
    if (!arg) {
        throw new TypeError("arg is invalid")
    }
}
try {
    assertIsTrue(false);
} catch (e) {
    // Type of "e" variable is "TypeError | unknown"
}

// TypeScript
function assertIsTrue(arg) {
    if (!arg) {
        throw new TypeError("arg is invalid")
    }
}
try {
    assertIsTrue(false);
} catch (e) {
    // Type of "e" variable is "any"
}

No new constructors

Unlike TypeScript, Hegel is not a superset language. That means constructors and features outside of JavaScript, such as decorators, private class fields, namespaces, enums, and other goodies from TypeScript, are not available in Hegel.

// TypeScript
enum UserStatus {
  Active,
  Muted,
  Banned
}
class User {
  constructor(
    public name: string,
    public status: UserStatus
  ) {}
}
const Anatoly = new User("Anatoly", UserStatus.Active);

// Hegel
const UserStatus = Object.freeze({
  Active: "Active",
  Muted: "Muted",
  Banned: "Banned"
});
class User {
    name: string;
    status: $Values<$TypeOf<UserStatus>>
    constructor(name, status) {
        this.name = name;
        this.status = status;
    }
}
const Anatoly = new User("Anatoly", UserStatus.Active);

No type coercion and any type

Since Hegel is concerned with implementing a sound type system, it doesn’t have type coercion or an any type.

// Error: There is no "any" type in Hegel.
const something: any = null;

// Error: Type cast does not exist in Hegel
(null: any).call();

Hegel and Flow

Hegel shares many similarities with Flow since they are both static type checker libraries. Below are some notable differences between Hegel and Flow.

#hegel #typescript #flow