Nest.JS | Monads -> IO

The IO monad, also known as the “Input/Output monad,” is a functional programming concept that allows you to separate the description of an input/output operation from its execution. In other words, it allows you to define a computation that performs some I/O action (such as reading from a file or interacting with a database) without actually performing that action until it is explicitly invoked.

The IO monad is often used to manage side-effects in a functional programming language, such as JavaScript. The IO monad allows you to describe an effectful computation in a pure, composable way, which makes it easier to reason about and test your code.

An IO monad is typically implemented as an object that has a single method, called run, which performs the I/O action and returns the result. The IO monad can be used to chain together multiple I/O operations and create more complex computations.

Here’s an example of the IO monad in JavaScript:

class IO {
  constructor(effect) {
    this.effect = effect
  }
  run() {
    return this.effect()
  }
  map(f) {
    return new IO(() => f(this.run()))
  }
  chain(f) {
    return new IO(() => f(this.run()).run())
  }
  // ...
}

Here’s an example of how you could use the IO Monad in Nest.js:

import { Injectable } from '@nestjs/common';
import { IO, io } from 'fp-ts/lib/IO';

@Injectable()
export class MyService {
  public async getUsers(): IO<User[]> {
    return io.fromPromise(this.userRepository.find());
  }

  public async getUserById(id: string): IO<User> {
    return io.fromPromise(this.userRepository.findOne(id));
  }
}

In this example, the MyService class has two methods getUsers() and getUserById(id:string) that returns an instance of the IO Monad which wraps a function that will resolve with an array of users or a user respectively.

You can chain multiple computations using the .map(), .chain(), .ap() methods.  

In this example, it filters the users that have age greater than 18, then maps the names of the remaining users, and finally, concatenates all the names using reduce method.

It’s important to note that this example is a basic one and doesn’t demonstrate the full power of Monads in functional programming. Monads can be used to handle various cases like error handling, chaining multiple computations and more, it’s best to understand the problem and use cases before applying monads.

Also, the run() method must be called in order to execute the IO Monad and get the results, this is known as impure as it's depend on the external state/context, IO Monads are used to separate the impure code from the pure code for better code organization and testing.