f# - FSharp.Core中未记录的 `when`关键字用法

Question

在寻找有关内联函数的静态解析类型参数的信息时，我偶然发现了FSharp.Core中various primitive operators的定义:

let inline (+) (x: ^T) (y: ^U) : ^V = 
     CheckedAdditionDynamic<(^T),(^U),(^V)>  x y 
     when ^T : int32       and ^U : int32      = (# "add.ovf" x y : int32 #)
     when ^T : float       and ^U : float      = (# "add" x y : float #)
     // <snip>
     when ^T : ^T = ((^T or ^U): (static member (+) : ^T * ^U -> ^V) (x,y))

在上面的代码段中可以看到 when关键字以以下格式使用: when expr1 = expr2用于各种内置类型。我猜想这相当于“如果T = int使用opcode add.ovf，否则，如果...，否则，则这样做”的某种编译器。

但是，我在F＃文档中找不到对这种语法的单一引用/说明。能够对F＃有所了解的人可以解释上面那段代码中的情况吗？

Answer 1

用户Carsten认为此答案有误，对此回答提供了以下评论。

the thing is: when used as is here has nothing to do with the documented usages - it seems to be called static conditional optimization and should not be used outside the core libraries - indeed go on and try to use it - you will see that you cannot unless you use the tricks mentioned in Johns answer (other question)

用户Carsten在此答案中添加了其他注释:

I added a comment - I don't think my educated guess is worth an answer - I hoped that one of the insiders hanging around would finally put an official answer to it

Carsten的第一条评论中提到的答案是用户John Palmer在2013年4月提出的，该链接与他在 (# ..... #)语法 What is the (# ... #) syntax seen in F3 standard library implementation?上提供的答案有关。

You can actually use this but you have to specify the --compiling-fslib (undocumented) and --standalone flags in your code.

用户MisterMetaphor提供了一个答案，引用了一个论坛中的帖子，内容如下:

Embedded IL in F# codes. Is this feature officially supported?

并不是的。此功能的99.9％目的是用于操作
在FSharp.Core.dll中定义(在1.9.2.9及更高版本中称为fslib.dll)。

有关 when关键字的其他用法，请参见以下内容。

本Microsoft文档介绍了使用 when关键字作为匹配条件 Match Expressions (F#)的其他条件。

本Microsoft文档介绍了如何使用 when关键字来表示对通用类型参数 Constraints (F#)的约束。

另请参阅此Microsoft文档，该文档描述了在各种设置 Pattern Matching (F#)中与 when关键字进行模式匹配的情况。

模式匹配文档说了以下内容以及几个示例。

Patterns are rules for transforming input data. They are used throughout the F# language to compare data with a logical structure or structures, decompose data into constituent parts, or extract information from data in various ways.

匹配表达式文档将以下内容与示例进行说明。

You can use a when clause to specify an additional condition that the variable must satisfy to match a pattern. Such a clause is referred to as a guard. The expression following the when keyword is not evaluated unless a match is made to the pattern associated with that guard.