abstract-class.ts

abstract class M<A> {
  abstract flatMap<B>(f: (a: A) => M<B>): M<B>;
}
abstract class MX<A> extends M<A> {
  abstract flatMap<B>(f: (a: A) => MX<B>): MX<B>; // if omitted: error TS2339: Property 'flatMap' does not exist on type 'MX<A>'.
  map<B>(f: (a: A) => B): MX<B> { return this.flatMap((a) => { return this.unit(f(a)) }) }
  unit<A>(a: A): MX<A> { return new MX<A>(a) }
}
class N<A> extends MX<A> {
  constructor(private value: A) {}
  flatMap<B>(f: (a: A) => N<B>): N<B> { return f(this.value) }
}