/**
 *****************************************
 * Created by edonet@163.com
 * Created on 2021-03-23 23:10:37
 *****************************************
 */
'use strict';


/**
 *****************************************
 * 元子函数
 *****************************************
 */
type P = (state: unknown) => unknown;


/**
 *****************************************
 * 函数参数
 *****************************************
 */
type A<T> = T extends (argv: infer P) => unknown ? P : never;


/**
 *****************************************
 * 函数返回值
 *****************************************
 */
type R<T> = T extends (argv: unknown) => infer P ? P : void;


/**
 *****************************************
 * 函数列表
 *****************************************
 */
type Fn<T> = (
    T extends [] ? [] :
    T extends [P] ? T :
    T extends [infer U, infer V] ? [U, (argv: R<U>) => R<V>] :
    T extends [...rest: infer W, argv1: infer U, argv2: infer V] ? [...Fn<[...W, U]>, (argv: R<U>) => R<V>] :
    P[]
);


/**
 *****************************************
 * 组合函数
 *****************************************
 */
type Flow<T> = (
    T extends [] ? () => void :
    T extends [infer U] ? U :
    T extends [head: infer U, ...rest: unknown[], tail: infer V] ? (argv: A<U>) => R<V> :
    () => void
);


/**
 *****************************************
 * 组合函数
 *****************************************
 */
export function flow<T extends P[]>(...args: Fn<T>): Flow<T> {

    // 校验参数
    args.forEach(handler => {
        if (typeof handler !== 'function') {
            throw new Error('expect handler to be a function!');
        }
    });

    // 定义函数
    function fn(state: unknown): unknown {
        return args.reduce((result, handler) => handler(result), state);
    }

    // 返回函数
    return fn as Flow<T>;
}