Nest.JS | Monads -> Identity

The Identity monad is a simple monad that wraps a value without applying any additional behavior or computation. It is often used as a building block for more complex monads.

The Identity monad is typically implemented as an object that has a single method, called run, which takes the wrapped value and returns it. The Identity monad can be used to wrap a value and chain together multiple computations that operate on the wrapped value.

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

class Identity {
  constructor(value) {
    this.value = value;
  }
  map(f) {
    return new Identity(f(this.value));
  }
  chain(f) {
    return f(this.value);
  }
  run() {
    return this.value;
  }
}

In this example, the Identity class takes a value in its constructor, and has methods map, chain and run that allow you to transform the wrapped value and chain multiple computations together.

For example, you could create an Identity that wraps a number and then use the map method to increment that number, you could then chain multiple of these Identities together to create a more complex computation, without having to manually unwrap the value at each step.

In Nest.js, a Monad is an object that wraps a value and provides a set of methods for working with that value in a functional way. The Identity Monad is a simple Monad that wraps a value and does not perform any additional operations on it.

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

import { Injectable } from '@nestjs/common';
import { Identity } from 'fp-ts/lib/Identity';

@Injectable()
export class MyService {
  public doSomething(): Identity<string> {
    return new Identity('Hello, World!');
  }
}

In this example, the MyService class has a method doSomething() that returns an instance of the Identity Monad, which wraps the string "Hello, World!".

To extract the wrapped value, you can use the .value property or the .extract() methodconsole.log(myService.doSomething().value)
console.log(myService.doSomething().extract())

Both of these will print ‘Hello, World!’

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.