matcher.ts

type IndexableKeys<T> = { [key in keyof T]: T[key] extends string ? key : never }[keyof T];

function matcher<T>(): <K extends IndexableKeys<T>>(key: K) => <R>(
  legs: {
    [key in Extract<T[Extract<K, keyof T>], string>]: (arg: T & Record<K, key>) => R
  }
) => (t: T) => R {
  return (key) => (legs) => (t) => {
    const state = t[key];
    return legs[state](t);
  }
}

function matcherOptional<T>(): <K extends IndexableKeys<T>>(key: K) => <R>(
  legs: {
    [key in Extract<T[Extract<K, keyof T>], string>]?: (arg: T & Record<K, key>) => R
  }
) => (t: T) => R | undefined {
  return (key) => (legs) => (t) => {
    const state = t[key];
    return legs[state]?.(t);
  }
}

type TypeWithStateAndAction = { action: 'kaki', state: 'hello', payload: { number: number } } | { action: 'pipi', state: 'goodbye', payload2: { string: string } };

const byAction = matcher<TypeWithStateAndAction>()("action")({
  kaki({ payload }) {
    return payload.number + 10;
  },
  pipi({ payload2 }) {
    return payload2.string.length;
  }
});

const byState = matcher<TypeWithStateAndAction>()("state")({
  goodbye({ payload2 }) {
    return `payload: ${payload2.string}`;
  },
  hello({ payload }) {
    return `number: ${payload.number}`;
  }
});

const byStateOptional = matcherOptional<TypeWithStateAndAction>()("state")({
  goodbye({ payload2 }) {
    return `payload: ${payload2.string}`;
  },
});

console.log(byAction({ action: "kaki", payload: { number: 10 }, state: 'hello' }));
console.log(byState({ action: "kaki", payload: { number: 10 }, state: 'hello' }));
console.log(byStateOptional({ action: "kaki", payload: { number: 10 }, state: 'hello' }));