F#将带有参数的运算符传递给函数 [英] F# passing an operator with arguments to a function

问题描述

您能否仅使用部分应用的运算符传递除以2"或减1"之类的运算，其中加1"如下所示:

Can you pass in an operation like "divide by 2" or "subtract 1" using just a partially applied operator, where "add 1" looks like this:

List.map ((+) 1) [1..5];;  //equals [2..6]
// instead of having to write: List.map (fun x-> x+1) [1..5]

正在发生的是将1作为第一个参数应用到(+)，并将列表项作为第二个参数应用.对于加法和乘法，此参数顺序无关紧要.

What's happening is 1 is being applied to (+) as it's first argument, and the list item is being applied as the second argument. For addition and multiplication, this argument ordering doesn't matter.

假设我想从每个元素中减去1(这可能是一个常见的初学者错误):

Suppose I want to subtract 1 from every element (this will probably be a common beginners mistake):

List.map ((-) 1) [1..5];;  //equals [0 .. -4], the opposite of what we wanted

1作为第一个参数应用于(-)，所以我得到的是(1 - list_item)而不是(list_item - 1).我可以将其重写为添加负数而不是减去正数:

1 is applied to the (-) as its first argument, so instead of (list_item - 1), I get (1 - list_item). I can rewrite it as adding negative one instead of subtracting positive one:

List.map ((+) -1) [1..5];;
List.map (fun x -> x-1) [1..5];; // this works too

我正在寻找一种更具表现力的书写方式，例如((-) _ 1)，其中_表示占位符，例如Arc语言.这将导致1是-的第二个参数，因此在List.map中它将求值为list_item - 1.因此，如果您想将divide by 2映射到列表，则可以编写:

I'm looking for a more expressive way to write it, something like ((-) _ 1), where _ denotes a placeholder, like in the Arc language. This would cause 1 to be the second argument to -, so in List.map, it would evaluate to list_item - 1. So if you wanted to map divide by 2 to the list, you could write:

List.map ((/) _ 2) [2;4;6] //not real syntax, but would equal [1;2;3] 
List.map (fun x -> x/2) [2;4;6] //real syntax equivalent of the above

可以做到这一点还是必须使用(fun x -> x/2)?似乎最接近占位符语法的是使用带有命名参数的lambda.

Can this be done or do I have to use (fun x -> x/2)? It seems that the closest we can get to the placeholder syntax is to use a lambda with a named argument.

推荐答案

您可以编写一个翻转函数，例如:

You could write a flip function, something like:

let flip f x y = f y x

List.map (flip (-) 1) [2;4;6]

我的语法可能不正确，我不太熟练F#.

I may have the syntax wrong, I'm not terribly fluent in F#.

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