monad.ts

Monad Structure for TypeScript

Monad.ts is a lib capable of giving to TypeScript the ability of having safe IO.
If you don’t know what a monad is, I recommend you to read about it.

install it with npm or yarn
yarn add @dudousxd/monad.ts
npm i @dudousxd/monad.ts

Use this lib is quite simple! You only need to import Monad and encapsulate your object inside it.

import Monad from '@dudousxd/monad.ts';
const myMonad: Monad<string> = Monad.of('Hello World');

When encapsulated, you have some methods to make operations on your monad.

bind(fn(x: any) => any): Monad<T>

This method takes a function as argument and returns a new Monad with your desired type.

const myMonad: Monad<number[]> = Monad.of([1, 2, 3, 4, 5]);
const f = (x: number) => x + 1;
const mappedMonad: Monad<number[]> = myMonad.bind(f);
console.log(mappedMonad.value) // [2, 3, 4, 5 , 6]

Since this method returns another Monad, you can chain methods like this:

const myMonad: Monad<number[]> = Monad.of([1, 2, 3, 4, 5]);
const f = (x: number) => x + 1;
const g = (x: number) => x * 2;
const mappedMonad: Monad<number[]> = myMonad.bind(f).bind(g);
console.log(mappedMonad.value) //[4, 6, 8, 10, 12]

And of course, you can change the type of your Monad based on your operations

const myMonad: Monad<number> = Monad.of(1);
const f = (x: number): string => x.toString();
const mappedMonad: Monad<string> = myMonad.bind(f);
console.log(typeof mappedMonad.value) //string

flatMap(fn(x: any) => any): Monad<T>

As you can see, bind and flatMap receives and returns the same stuff, there is only one little diference between them: While map
operates in a single object (like an array of objects), flatMap can take several values (like an array of arrays), join them in a
single object and then operate, e.g:

const myMonad: Monad<number[][]> = Monad.of([[1], [2, 3], [4, 5, 6]]);
const f = (x: number) => x + 1;
const mappedMonad: Monad<number[]> = myMonad.flatMap(f);
console.log(mappedMonad.value) //[2, 3, 4, 5, 6, 7]

If your array has any empty array, it will be unnoticed

const myMonad: Monad<number[][]> = Monad.of([[1], [], [4, 5, 6]]);
const f = (x: number) => x + 1;
const mappedMonad: Monad<number[]> = myMonad.flatMap(f);
console.log(mappedMonad.value); // [2, 5, 6, 7]

Aaand we can’t forget, you can chain flatMap with bind

const myMonad: Monad<number[][]> = Monad.of([[1], [], [4, 5, 6]]);
const f = (x: number) => x + 1;
const g = (x: number) => x * 2;
const mappedMonad: Monad<number[]> = myMonad.flatMap(f).bind(g);
console.log(mappedMonad.value); // [4, 6, 8, 10, 12, 14]

either(e: Error, monadValue: any):any

This either function is like a catch. You can define what the monad will do in case your operation throws something. This either function has to return something, otherwhise it will stop your chain.
Either function receives the error and the Monad value of the bind that just throwed an error, so you do something in case of error.

(error: Erorr, monadValue):any => {console.log(e); return []}

This is the default either. It will log your error and return an empty array, e.g:

for this example, we’ll create two interfaces:

interface SonJson{ 
  name: string;
  parents: ParentsJson;
}
interface ParentsJson {
  dad: string;
  mother: string
}

const myMonad: Monad<SonJson> = Monad.of({name: 'Son', parents: {dad: 'Dad', mother:'Mother'}});
const f = (x: SonJson):ParentsJson => x.parents;
const g = (x: any):string => x.test;
const h = (x: string):number => x.length;
const mappedMonad = myMonad.bind(f).bind(g).bind(h);
console.log(mappedMonad.value); // []

In the example above, g function will look for an attribute called test. Since this attribute doesn’t exist, it will return undefined. After that, h function will try to get the length of undefined, so it will give you an error. To stop the error from spreading and stoping our application, either will return an empty array.

Either will work and make your bindings go all the way until the end even if an error happened on your third bind and you have 4 binds, like this:

const myMonad: Monad<SonJson> = Monad.of({name: 'Son', parents: {dad: 'Dad', mother:'Mother'}});
const f = (x: SonJson):ParentsJson => x.parents;
const g = (x: any):string => x.teste;
const h = (x: string):number => x.length;
const j = (x: number):number => x*x
const mappedMonad = myMonad.bind(f).bind(g).bind(h).bind(j);
console.log(mappedMonad.value) // []

To define a custom either function, you only need to do:

const customEither = (error, monadValue) => {console.log('customEither'); return ''};
const myMonad: Monad<SonJson> = Monad.of({name: 'Son', parents: {dad: 'Dad', mother:'Mother'}}).setEither(customEither);
const f = (x: SonJson):ParentsJson => x.parents;
const g = (x: any):string => x.teste;
const h = (x: any):number => x.length;
const mappedMonad = myMonad.bind(f).bind(g).bind(h);
console.log(mappedMonad.value); // ''

You can also call setEither in the middle of a chain of bindings, this is really helpful if you want diferent either functions for different bindings

const customEitherOne = (error, monadValue) => {console.log('customEitherOne'); return ''};
const customEitherTwo = (error, monadValue) => {console.log('customEitherTwo'); return ''};
const customEitherThree = (error, monadValue) => {console.log('customEitherThree'); return []};
const myMonad: Monad<SonJson> = Monad.of({name: 'Son', parents: {dad: 'Dad', mother:'Mother'}}).setEither(customEither);
const f = (x: SonJson):ParentsJson => x.parents;
const g = (x: any):string => x.teste;
const h = (x: any):number => x.length;
const mappedMonad = myMonad
                    .setEither(customEitherOne)
                    .bind(f)
                    .setEither(customEitherTwo)
                    .bind(g)
                    .setEither(customEitherThree)
                    .bind(h);
console.log(mappedMonad.value); // []

If you need, you can set either function back to default behavior with setDefaultEither

const customEitherOne = (error, monadValue) => {console.log('customEitherOne'); return ''};
const customEitherTwo = (error, monadValue) => {console.log('customEitherTwo'); return ''};
const myMonad: Monad<SonJson> = Monad.of({name: 'Son', parents: {dad: 'Dad', mother:'Mother'}}).setEither(customEither);
const f = (x: SonJson):ParentsJson => x.parents;
const g = (x: any):string => x.teste;
const h = (x: any):number => x.length;
const mappedMonad = myMonad
                    .setEither(customEitherOne)
                    .bind(f)
                    .setEither(customEitherTwo)
                    .bind(g)
                    .setDefaultEither()
                    .bind(h);
console.log(mappedMonad.value); // []

Important!

It’s important to remember that an either function will be passed to all the other binds, so if you set an either function that returns something valid only for my first bind, the Monad will return the either function to the default behavior

const customEither = (e: any, monadValue: any) => {
  console.error('customEither');
  return 3;
};
const myMonad: Monad<SonJson> = Monad
                                .of({name: 'Son', parents: {dad: 'Dad', mother: 'Mother'}})
                                .setEither(customEither);
const f = (x: SonJson): ParentsJson => x.parents;
const g = (x: any): string => x.test;
const h = (x: any): number => x.length; // it will return 3
const i = (x: string): string => x.toUpperCase(); // it will try to uppercase 3, this will blow an error, so instead, monad will return the default behavior, in other words, will return []
const mappedMonad = myMonad
                    .bind(f)
                    .bind(g)
                    .bind(h)
                    .bind(i);
console.log(mappedMonad.value); // []

Monad Types

Since Monads can throw error and stop your bind chain (like the cases we saw on either), they have two types to tell if everything ocurred okay or not:

• Just: Monad is a Maybe if your value on Monad.of(value) is a valid value or your operations ocurred until the end
• Nothing: If you pass a falsey value for Monad.of, like null, or an empty array, your Monad will be Nothing. If an error occured during any of your bindings, the result of your Monad will be a Nothing Monad.

To get the type of your Monad, you can use the method getMonadType(): string, and it will return the Monad type as a string.