TypeScript: Pattern Matching & Type Safety

Mastering Type Safety in TypeScript: A Guide to Algebraic Data Types and Pattern Matching

Type safety is a fundamental concept in programming, and TypeScript provides a robust type system to ensure that your code is correct and maintainable. In this article, we’ll explore algebraic data types and pattern matching, two powerful techniques for improving type safety in your TypeScript applications.

What are Algebraic Data Types?

Algebraic data types (ADTs) are a way of combining multiple types into a single type. They’re useful for modeling complex data structures and for ensuring that your code handles all possible cases. In TypeScript, you can create ADTs using the type keyword.

typescript
type Either<T> = {
tag: 'left';
error: T;
} | {
tag: 'right';
value: T;
};


In this example, we’ve created an Either type that can be either a left value with an error or a right value with a result.

Pattern Matching with Switch Statements

Once you have an ADT, you can use pattern matching to handle different cases. In TypeScript, you can use switch statements to pattern match on the tag of an ADT.

typescript
function match<T>(value: Either<T>): string {
switch (value.tag) {
case 'left':
return `Error: ${value.error}`;
case 'right':
return `Result: ${value.value}`;
default:
throw new Error('Invalid tag');
}
}


In this example, we’ve created a match function that takes an Either value and returns a string based on the tag.

Pattern Matching Libraries

While switch statements are a good way to pattern match, they can become cumbersome for more complex cases. Fortunately, there are several pattern matching libraries available for TypeScript.

• TS-Pattern: TS-Pattern is a lightweight library that provides a simple and expressive way to pattern match.
• Unionize: Unionize is a library that focuses on creating tagged unions and action creators with minimal boilerplate.
• Pratica: Pratica is a functional programming library that provides tools for creating and working with sum and product types.

Here’s an example of using TS-Pattern to pattern match on an Either value:

“`typescript
import { match } from ‘ts-pattern’;

const value: Either = { tag: ‘right’, value: ‘hello’ };

const result = match(value)
.with({ tag: ‘left’ }, () => ‘Error’)
.with({ tag: ‘right’ }, () => ‘Result’)
.run();

console.log(result); // Output: Result
“`

Type-Safe Reducers

ADTs and pattern matching are also useful for creating type-safe reducers. A reducer is a function that takes an action and returns a new state. By using ADTs and pattern matching, you can ensure that your reducer handles all possible actions.

“`typescript
type Action = {
type: ‘INCREMENT’;
} | {
type: ‘DECREMENT’;
};

type State = number;

function reducer(state: State, action: Action): State {
switch (action.type) {
case ‘INCREMENT’:
return state + 1;
case ‘DECREMENT’:
return state – 1;
default:
throw new Error(‘Invalid action’);
}
}
“`

In this example, we’ve created a reducer that takes an action and returns a new state. We’ve used an ADT to model the action and pattern matching to handle different cases.

Runtime Types

Finally, let’s talk about runtime types. Runtime types are a way of checking the type of a value at runtime. They’re useful for ensuring that your code handles all possible cases.

In TypeScript, you can use the io-ts library to create runtime types.

“`typescript
import * as t from ‘io-ts’;

const User = t.type({
name: t.string,
age: t.number,
});

const user = { name: ‘John’, age: 30 };

const result = User.decode(user);

if (result.isRight()) {
console.log(‘User is valid’);
} else {
console.log(‘User is invalid’);
}
“`

In this example, we’ve created a User type using the io-ts library. We’ve then used the decode method to check if a user object is valid.

By using ADTs, pattern matching, and runtime types, you can ensure that your TypeScript code is correct and maintainable. These techniques are especially useful for creating complex data structures and for handling all possible cases.