下面的F＃code ++工程，如我所料，印刷`匹配为A：

The following F# code works as I expected, printing `Matched as 'A':

let (|Char|_|) convf = function
    | LazyList.Nil -> None
    | LazyList.Cons (x, _) -> Some (convf x)

let test = function
    | Char System.Char.ToUpper x -> printfn "Matched as %A" x
    | _ -> printfn "Didn't match"

test (LazyList.of_list ['a'])

不过，如果我更改字符从部分活跃的方式，以一个完整的活动模式如下：

However, if I change Char from a partial active pattern to a complete active pattern as follows:

let (|Char|NoChar|) convf = function
    | LazyList.Nil -> NoChar
    | LazyList.Cons (x, _) -> Char x

let test = function
    | Char System.Char.ToUpper x -> printfn "Matched as %A" x
    | NoChar System.Char.ToUpper -> printfn "Didn't match"

test (LazyList.of_list ['a'])

那么code编译失败，给下面的错误信息：错误FS0191：只有激活模式返回只有一个结果是可以接受的参数

这个例子可能看起来有点做作，但它是一个活跃的格局我试图在Prolog的词法分析器我一直在我的业余时间使用的简化版本。我可以很容易地重写我的code，以避免这个问题，但我很好奇，为什么这样的code是不允许的。

This example may look somewhat contrived, but it's a simplified version of an active pattern I tried to use in a Prolog lexer I've been working on in my spare time. I can easily rewrite my code to avoid this problem, but I'm curious about why this sort of code is disallowed.

更新：的较新版本的F＃似乎已经改名为这个错误：

Update: the newer versions of F# seem to have renamed this error:

错误FS0722：只有激活模式返回只有一个结果是可以接受的参数

推荐答案

NB。这正是Brian说，但在一个更清晰的方式希望说明。的

我记得登录precisely这个问题上的错误，并IIRC这是唐赛姆不得不说的事。

I recall logging a bug on precisely this issue and IIRC this is what Don Syme had to say on the matter.

一个多情况下，主动模式是从一​​些输入值的转换功能分为多个输出值中的一个。在你的榜样，任何字符转换为字符的情况下或NoChar情况。

A multi-case active pattern is a conversion function from some input value into one of several output values. In your example, any character is converted to the Char case or the NoChar case.

这样做的好处是，F＃编译器调用多情况下，主动模式功能，一次就可以大致判断明年评估哪种模式匹配规则。

The benefit of this is that the F# compiler calls the multi-case active pattern function once and can then generally determine which pattern match rule to evaluate next.

然而，如果你让一个参数，那么你需要评估多情况下，主动模式的每一个模式匹配规则。

If you allow a parameter however, then you need to evaluate the multi-case active pattern for every pattern match rule.

所以，想象一下以下

match input with
| Alpha "foo" -> ...
| Bravo "bar" -> ...

在评估（|阿尔法|布拉沃|）富返回布拉沃，那么第一个规则不匹配。 Likeways（|阿尔法|布拉沃|）栏中的返回阿尔法，那么第二条规则就不是非此即彼匹配。所以，你真的没有多的情况下主动模式。就在paramterized，局部活跃格局。 （由于一些输入的预期图案情况下也不会打了。）

When evaluating (|Alpha|Bravo|) "foo" returned 'Bravo', then the first rule wouldn't match. Likeways (|Alpha|Bravo|) "bar" returns 'Alpha', then the second rule wouldn't match either. So you don't really have a multi-case active pattern. Just a paramterized, partial active pattern. (Because for some inputs the expected pattern-case won't be hit.)

因此​​，当用不构成一个整体非常有意义的语言的一个角落里遇到，实际上可更清晰的部分，参数化的活动模式。该功能未添加到语言

So when confronted with a corner of the language that doesn't make a whole lot of sense, and in fact can be made much clearer by the partial, parameterized active pattern. The feature wasn't added to the language.