monad

monad.ts

// Define the Monad structure using `unit` and `bind`
type Monad<M extends { __type: any }> = {
  unit: <A>(value: A) => M & { __type: A };
  bind: <A, B>(ma: M & { __type: A }, f: (a: A) => M & { __type: B }) => M & { __type: B };
};

// Example: Maybe monad implementation
type Just<A> = { __type: A; tag: 'Just'; value: A };
type Nothing = { __type: any; tag: 'Nothing' };
type Maybe<A> = Just<A> | Nothing;

const Maybe = {
  unit: <A>(value: A): Maybe<A> => ({ __type: value as A, tag: 'Just', value }),
  nothing: { __type: undefined, tag: 'Nothing' } as Nothing,
};

const MaybeMonad: Monad<Maybe<any>> = {
  unit: <A>(value: A) => Maybe.unit(value),
  bind: <A, B>(ma: Maybe<A>, f: (a: A) => Maybe<B>) => 
    ma.tag === 'Just' ? f(ma.value) : Maybe.nothing,
};

// Usage:
const result = MaybeMonad.bind(
  MaybeMonad.unit(2),
  (n: number) => MaybeMonad.unit(n * 3) // Returns Just(6)
);