Maybe and Either type implementation in TypeScript

maybe-either.ts

type ADT0<T> = [T]
type ADT1<T, V> = [T, V]

// ---------------------------------
type Just<T>  = ADT1<'Just:', T>
type Nothing  = ADT0<'Nothing:'>
type Maybe<J> = Nothing | Just<J>
// functions
const just      = <T>(v: T): Maybe<T> => ['Just:', v];
const nothing   = <T>():Maybe<T> => ['Nothing:'];
const isJust    = <T>(v: Maybe<T>): v is Nothing => (v[0] === 'Just:');
const isNothing = <T>(v: Maybe<T>): v is Just<T> => (v[0] === 'Nothing:');

// ---------------------------------
type Left<T>     = ADT1<'Left:', T>;
type Right<T>    = ADT1<'Right:', T>
type Either<L,R> = Left<L> | Right<R>
// functions
const left    = <L,R>(v: L):Either<L, R> => ['Left:', v];
const right   = <L,R>(v: R):Either<L, R> => ['Right:', v];
const isLeft  = <L,R>(v: Either<L, R>): v is Left<L> => (v[0] === 'Left:');
const isRight = <L,R>(v: Either<L, R>): v is Right<R> => (v[0] === 'Right:');

// --------------------------------
const type = <T>(x: [T, ...any]):T => x[0];
// --------------------------------

const x: Either<number, Maybe<string>> = right(just('Hello!'));
const y: Either<number, Maybe<string>> = left(3);

console.log(x);                    // [ 'Right:', [ 'Just:', 'Hello!' ] ]
console.log(y);                    // [ 'Left:', 3 ]
console.log(isLeft(x));            // false
console.log(isLeft(y));            // true
console.log(isRight(x));           // true
console.log(isRight(y));           // false
console.log(type(x) === 'Left:');  // false
console.log(type(y) === 'Right:'); // false