爲什麼Seq.iter和Seq.map如此之慢？

Question

考慮這個代碼在F＃：

let n = 10000000 
let arr = Array.init n (fun _ -> 0) 

let rec buildList n acc i = if i = n then acc else buildList n (0::acc) (i + 1) 
let lst = buildList n [] 0 

let doNothing _ =() 
let incr x = x + 1 

#time 

arr |> Array.iter doNothing   // this takes 14ms 
arr |> Seq.iter doNothing   // this takes 74ms 

lst |> List.iter doNothing   // this takes 19ms 
lst |> Seq.iter doNothing   // this takes 88ms 

arr |> Array.map incr    // this takes 33ms 
arr |> Seq.map incr |> Seq.toArray // this takes 231ms! 

lst |> List.map incr    // this takes 753ms 
lst |> Seq.map incr |> Seq.toList // this takes 2111ms!!!! 

爲什麼Seq模塊上的iter和map功能比Array和List模塊等同這麼多慢？

Answer 1

一旦您撥打Seq，您將丟失類型信息 - 移至列表中的下一個元素需要致電IEnumerator.MoveNext。與Array相比，您只需增加一個索引，而對於List，則只需取消引用一個指針即可。實質上，您將獲得列表中每個元素的額外函數調用。

的轉換回List和Array也慢了下來代碼出於類似的原因