如何实现串行promise

// 模拟ajax
const ajax = (id,timeout=500) => () => new Promise(resolve=>setTimeout(resolve.bind(null,id), timeout));

// arr:参数数组
// fn:对每个参数需要执行的函数
const main = (arr,fn)=>{
    const [values,reasons] = [[],[]];
    return new Promise(resolve=>{
        arr
         .reduce((total,cur)=>(
            total
             .then(fn(cur))
             .then(
                 // 如果不用bind，数组的push方法中的this会丢失
                 values.push.bind(values),
                 reasons.push.bind(reasons)
             )
         ),Promise.resolve())
         .then(resolve.bind(null,[values,reasons]))
    })

}

main([1,2,4,4,5,6,7],ajax).then(console.log)