this compiles !???

gistfile1.ts

interface Functor<A>{
  flatMap<A,B> (f: (A) => B): Functor<B>
}

interface Monad<A> extends Functor<A> {
  bind: (A) => Monad<A>
}

interface Option<A> extends Monad<A>{ 
}
 
class Some<A> implements Option<A> {
  private value: A
  constructor(a: A) {
    this.value = a
  }
  flatMap<A,B> (f: (A) => B): Some<B>{
    return new Some(f(this.value))
  }
  bind (f: (A) => Some<A>): Some<A>{
    return f(this.value)
  }
}

class None<A> implements Option<A>{
  flatMap<A,B> (f: (A) => B): None<B> {
    return new None<B>()
  }
  bind (f: (A) => None<A>): None<A>{
    return this
  }
}

var test:Option<string> = new Some(4)
test = new None()
test = new Some(false) // ! \o/ this compiles...