reader monad

reader.ts

type T_UnaryFunction<I, O> = (x: I) => O;
type T_Fn<I, O> = {
  run: T_UnaryFunction<I, O>;
  chain: <P>(f: T_UnaryFunction<O, T_Fn<I, P>>) => T_Fn<I, P>;
  map: <A>(f: T_UnaryFunction<O, A>) => T_Fn<I, A>;
};

const Fn = <I, O = I>(run: T_UnaryFunction<I, O>): T_Fn<I, O> => ({
  run,
  chain: <P>(f: T_UnaryFunction<O, T_Fn<I, P>>) => Fn((x) => f(run(x)).run(x)),
  map: <P>(f: T_UnaryFunction<O, P>) => Fn((x: I) => f(run(x))),
});

Fn.ask = <T>() => Fn((x:T) => x);
Fn.of = <T>(x: T) => Fn((_) => x);

const res = Fn<string>(x => x.toUpperCase())
.chain((upper) => Fn.ask<string>().map((lower) => [upper, lower]))
.run('hello');