Composite functions for TypeScript

chain.ts

type F<A, B> = (x: A) => B;

const chain = <A, B>(fab: F<A, B>) => {
  return {
    chain: <C>(fbc: F<B, C>) => chain((x: A) => fbc(fab(x))),
    value: fab,
  };
};

// example

const fromString = (s: string) => Number(s) || null;
const plus1 = (n: number) => n + 1;
const double = (n: number) => n * 2;
const print = (n: unknown) => `value is: ${n}`;

const notNull = <I, O>(f: F<I, O>) => (x: I | null) => {
  return x === null ? null : f(x);
};

const math = chain(plus1).chain(double).value;

export const f = chain(fromString)
.chain(notNull(math))
.chain(notNull(plus1))
.chain(print)
  .value;

console.log(f('42'));
console.log(f('wow'));