F# For 循环负数倒数 - int 类型不支持运算符..-

Question

我尝试在非递归阶乘函数中从 6 倒数到 1，但出现编译器错误:

let fact x =
    let mutable product = 1
    for i in x..-1..1 do
        product <- product * i
    product

// Error on the 6 - type 'int' does not support the operator '..-'
let answer = fact 6
printfn "%i" answer

我从底部附近得到了这个想法here

已将函数更改为仅计数并且它有效，但我有兴趣知道为什么会失败。有没有更好的倒数方法？

使用VS2012更新3

Answer 1

Have changed the function to just count up and it works, but I'm interested to know why this failed.

您的示例失败，因为 .. 和 - 是两个不同的运算符，编译器需要将它们分开。您可以添加空格，而不是用括号将 -1 括起来:

let fact x =
    let mutable product = 1
    for i in x .. -1 .. 1 do
        product <- product * i
    product

Is there a better way to count down?

不太为人所知的 for .. downto .. do 结构在这里使用更合适。

let fact x =
    let mutable product = 1
    for i = x downto 1 do
        product <- product * i
    product