Eq Type Class

eq.ts

// Eq type class
// => used to compare two inputs and check for equality
// Mostly taken from https://dev.to/gcanti/getting-started-with-fp-ts-setoid-39f3

// Our interface for the Eq type class
// For something to be a member of the Eq type class it has to implement an equals method that returns true
interface Eq<A> {
  // `true` if `x` equals `y`
  readonly equals: (x: A, y: A) => boolean;
}
 
// We create a Eq<number> instance
const eqNumber: Eq<number> = {
  equals: (x, y) => x === y
}

// contramap takes in a function that takes in something of type B and returns something of type A
// It then returns a function that takes in something of type Eq<A> and returns a new Eq<B>
const contramap = <A, B>(f: (b: B) => A) => (E: Eq<A>): Eq<B> => {
  return {
    equals: (x, y) => E.equals(f(x), f(y))
  }
}

type Artist = {
  artistId: number;
  name: string;
};
  
// Two artists are equal if their `artistId` field is equal
const eqArtist: Eq<Artist> = contramap((artist: Artist) => artist.artistId)(eqNumber);

// We can now pass in two Artist objects and check for equality
eqArtist.equals({ artistId: 1, name: "EDEN" }, { artistId: 1, name: "eden" }); // true
eqArtist.equals({ artistId: 1, name: "ATO" }, { artistId: 2, name: "ATO" }); // false