当使用“何时"时，F#不完整模式与此表达式匹配.为什么? [英] F# Incomplete pattern matches on this expression when using "when"..Why?

问题描述

我有这个简单的F#函数:

I have this simple F# function:

let compareNum x =
    let y = 10
    match x with
    | _ when x = y -> 0
    | _ when x > y -> 1
    | _ when x < y -> -1

但是，F#编译器给我此表达式上的模式匹配不完整"警告.在这种情况下，所有情况都应涵盖所有模式.

However, F# compiler gives me "Incomplete pattern matches on this expression" warning. In this case, all cases should cover every pattern.

在Chris Smith的《 Programming F#》第一版中的模式匹配"部分中，我也看到了类似的示例.因此，在更高版本的F#中可能会有所更改吗?

I also see a similar example in "Pattern Matching" section in the 1st edition of Programming F# book by Chris Smith. So something might be changed in the later version of F#?

推荐答案

我认为评论-通常，在最后一个模式中设置警惕是一种反模式" - by

I think the answer to the previous question (and the comments -- "In general, it is an anti-pattern to have a when guard in the last pattern" -- by kimsk) explain the situation.

但是，我不会说在最后一个模式中有一个守卫是一个反模式-这是最简单的解决方法，但是我发现这有点不幸，因为when模式提供了您将获得有关您可以期望的值的有用信息-从而使对该程序的理解更加容易.上次遇到此问题时，我至少把它留在了那里，至少是作为评论:

However, I would not say that having a guard in the last pattern is an anti-pattern - it is the easiest workaround, but I find this somewhat unfortunate, because the when pattern gives you useful information about the values you can expect - and that makes understanding the program easier. Last time I had this problem, I left it there, at least as a comment:

let compareNum x =
  let y = 10
  match x with
  | _ when x = y -> 0
  | _ when x > y -> 1
  | _ (*when x < y*) -> -1

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