为什么要在涉及方法应用的连续参数中加上括号? [英] Why should successive arguments involving method application be parenthesized?

问题描述

假设以下F#函数:

let f (x:int) (y:int) = 42

我怀疑我需要在下面的示例z2中加括号的原因是由于类型推断；我的例子可能不是很好，但是很容易想象事情会变得很毛茸茸:

I suspect that the reason I need to parenthesize the arguments in example z2 below is because of type inference; my example might not be great, but it's easy to imagine how things could get very hairy:

let z1 = f 2 3
let z2 = f 2 (f 3 5)

但是，以下情况我不太清楚:

However, the following case is less clear to me:

let rng = System.Random()
let z3 = f 1 rng.Next(5)

z3无法正常工作，并显示明确的错误消息:

z3 doesn't work, with a clear error message:

错误FS0597:后续参数应该用空格或 元组，以及涉及函数或方法应用程序的参数 加上括号.

error FS0597: Successive arguments should be separated by spaces or tupled, and arguments involving function or method applications should be parenthesized.

修复起来很琐碎(将所有内容括起来)，但是我不清楚的是为什么这样的表达式是一个问题.我认为这再次与类型推断有关，但是在我看来，天真地是，在此方法中，用括号括起来的参数列表实际上会使事情变得不太可能含糊不清.这是否与rng.Next(5)等同于rng.Next 5的事实有关?

Fixing it is trivial (parenthesize all the things), but what I am not clear about is why such an expression is a problem. I assume this has to do with type inference again, but naively, it seems to me that here, methods having a list of arguments surrounded by a parenthesis would actually make things less potentially ambiguous. Does this have to do with the fact that rng.Next(5) is equivalent to rng.Next 5?

有人可以提示，给出示例或解释为什么需要此规则，或者如果不存在该规则会出现什么类型的问题?

Can someone hint, give an example or explain why this rule is needed, or what type of problems would arise if it were not there?

推荐答案

我认为这里的问题是代码可以被视为:

I think that the problem here is that the code could be treated as:

let z3 = f 1 rng.Next (5)

这等效于省略括号，因此它将使用3个参数(第二个是函数值)调用f.这听起来有些愚蠢，但是编译器实际上并不严格要求在参数之间留有空格.例如:

This would be equivalent to omitting the parentheses and so it would be calling f with 3 arguments (the second being a function value). This sounds a bit silly, but the compiler actually does not strictly insist on having a space between parameters. For example:

let second a b = b
add 5(1)          // This works fine and calls 'add 5 1'
add id(1)         // error FS0597
add rng.Next(5)   // error FS0597
add (rng.Next(5)) // This works fine (partial application)

我认为问题在于，如果您看一下上面片段中的4个示例的顺序，则不清楚在第二种情况和第三种情况下应该获得哪种行为.

I think the problem is that if you look at the sequence of the 4 examples in the above snippet, it is not clear which behavior should you get in the second and the third case.

仍然以特殊方式处理呼叫rng.Next(5)，因为如果由单参数应用程序形成的呼叫没有空间，则F#允许您将其链接起来.例如rng.Next(5).ToString().但是，例如，允许写second(1)(2)，但是second(1)(2).ToString()将不起作用.

The call rng.Next(5) is still treated in a special way, because F# allows you to chain calls if they are formed by single-parameter application without space. For example rng.Next(5).ToString(). But, for example, writing second(1)(2) is allowed, but second(1)(2).ToString() will not work.

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