Scala中的多个参数列表和每个列表的多个参数之间有什么区别? [英] What's the difference between multiple parameters lists and multiple parameters per list in Scala?

问题描述

在Scala中，人们可以像这样编写(咖喱?)函数

In Scala one can write (curried?) functions like this

def curriedFunc(arg1: Int) (arg2: String) = { ... }

上面的具有两个参数列表的curriedFunc函数定义与一个参数列表中具有多个参数的函数之间的区别是什么?

What is the difference between the above curriedFunc function definition with two parameters lists and functions with multiple parameters in a single parameter list:

def curriedFunc(arg1: Int, arg2: String) = { ... }

从数学的角度来看，这是(curriedFunc(x))(y)和curriedFunc(x,y)，但是我可以写def sum(x) (y) = x + y，而它也是def sum2(x, y) = x + y

From a mathematical point of view this is (curriedFunc(x))(y) and curriedFunc(x,y) but I can write def sum(x) (y) = x + y and the same will be def sum2(x, y) = x + y

我只知道一个区别-这是部分应用的功能.但是这两种方式对我来说都是等效的.

I know only one difference - this is partially applied functions. But both ways are equivalent for me.

还有其他区别吗?

推荐答案

严格来说，这不是一个咖喱函数，而是具有多个参数列表的方法，尽管它看起来像一个函数.

Strictly speaking, this is not a curried function, but a method with multiple argument lists, although admittedly it looks like a function.

正如您所说，多个参数列表允许使用该方法代替部分应用的函数. (对不起，我使用的一般愚蠢的示例)

As you said, the multiple arguments lists allow the method to be used in the place of a partially applied function. (Sorry for the generally silly examples I use)

object NonCurr {
  def tabulate[A](n: Int, fun: Int => A) = IndexedSeq.tabulate(n)(fun)
}

NonCurr.tabulate[Double](10, _)            // not possible
val x = IndexedSeq.tabulate[Double](10) _  // possible. x is Function1 now
x(math.exp(_))                             // complete the application

另一个好处是，您可以使用花括号代替括号，如果第二个参数列表由单个函数或thunk组成，则括号看起来不错.例如

Another benefit is that you can use curly braces instead of parenthesis which looks nice if the second argument list consists of a single function, or thunk. E.g.

NonCurr.tabulate(10, { i => val j = util.Random.nextInt(i + 1); i - i % 2 })

与

IndexedSeq.tabulate(10) { i =>
  val j = util.Random.nextInt(i + 1)
  i - i % 2
}

或者是笨蛋:

IndexedSeq.fill(10) {
  println("debug: operating the random number generator")
  util.Random.nextInt(99)
}

另一个优点是，您可以引用先前的参数列表中的参数来定义默认参数值(尽管您也可以说无法在单个列表中执行此操作是一个缺点:)

Another advantage is, you can refer to arguments of a previous argument list for defining default argument values (although you could also say it's a disadvantage that you cannot do that in single list :)

// again I'm not very creative with the example, so forgive me
def doSomething(f: java.io.File)(modDate: Long = f.lastModified) = ???

最后，在相关帖子的答案中还有其他三个应用程序

Finally, there are three other application in an answer to related post Why does Scala provide both multiple parameters lists and multiple parameters per list? . I will just copy them here, but the credit goes to Knut Arne Vedaa, Kevin Wright, and extempore.

首先:您可以有多个var args:

First: you can have multiple var args:

def foo(as: Int*)(bs: Int*)(cs: Int*) = as.sum * bs.sum * cs.sum

...在单个参数列表中是不可能的.

...which would not be possible in a single argument list.

第二，它有助于类型推断:

Second, it aids the type inference:

def foo[T](a: T, b: T)(op: (T,T) => T) = op(a, b)
foo(1, 2){_ + _}   // compiler can infer the type of the op function

def foo2[T](a: T, b: T, op: (T,T) => T) = op(a, b)
foo2(1, 2, _ + _)  // compiler too stupid, unfortunately

最后，这是拥有隐式和非隐式参数的唯一方法，因为implicit是整个参数列表的修饰符:

And last, this is the only way you can have implicit and non implicit args, as implicit is a modifier for a whole argument list:

def gaga [A](x: A)(implicit mf: Manifest[A]) = ???   // ok
def gaga2[A](x: A, implicit mf: Manifest[A]) = ???   // not possible

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