export type BasicFun<a, b> = (_: a) => b;

export type Fun<a, b> = BasicFun<a, b> & {
  then<c>(other: BasicFun<b, c>): Fun<a, c>;
};

export const Fun = <a, b>(_: BasicFun<a, b>): Fun<a, b> =>
  Object.assign(_, {
    then: function <c>(this: Fun<a, b>, other: BasicFun<b, c>): Fun<a, c> {
      return Fun((_) => other(this(_)));
    },
  });

export type BasicFun2<a, b, c> = (_: a, __: b) => c;

export type Fun2<a, b, c> = BasicFun2<a, b, c> & {
  then<d>(other: BasicFun<c, d>): Fun2<a, b, d>;
};
export const Fun2 = <a, b, c>(_: BasicFun2<a, b, c>): Fun2<a, b, c> =>
  Object.assign(_, {
    then: function <d>(
      this: Fun2<a, b, c>,
      other: BasicFun<c, d>,
    ): Fun2<a, b, d> {
      return Fun2((a, b) => other(this(a, b)));
    },
  });

/*
// samples
const incr: Fun<number,number> = Fun(_ => _ + 1)
const decr: Fun<number,number> = Fun(_ => _ - 1)
const doub: Fun<number,number> = Fun(_ => _ * 2)
const halv: Fun<number,number> = Fun(_ => _ / 2)

const f = incr.then(doub).then(decr)
console.log(f(5)) // prints 11
*/