Scala 并行赋值仅在声明中

发布于 2024-11-03 01:38:15 字数 287 浏览 0 评论 0原文

有：

def f () = {
    (1, "two", 3.0)
}

为什么可以

var (x, y, z) = f()

但不行


var i = 0
var j = "hello"
var k = 0.0

// use i, j, k
...
//then
(i, j, k) = f() // ; expected but = found

？

原文

Having:

def f () = {
    (1, "two", 3.0)
}

Why is it ok

var (x, y, z) = f()

but not


var i = 0
var j = "hello"
var k = 0.0

// use i, j, k
...
//then
(i, j, k) = f() // ; expected but = found

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凉城凉梦凉人心 2024-11-10 01:38:15

您可以在此处看到初始化变量时模式匹配的有限版本。请注意，这不仅适用于元组：

val a :: b = List(1,2,3)
println(a) //1
println(b) //List(2, 3)

此功能似乎是直接从 Haskell 借用的，您也可以使用模式进行初始化：

let (a,b) = getTuple 
in a*b

由于 Haskell 没有可变数据，因此无法分配某些内容。

在 Scala 中，你可以做这样的事情，但我猜这被认为太混乱，或者可能太难实现。您始终可以像往常一样使用 match 表达式，并且通常您只需要一个 case，例如 List((1,2),(3,4)) .map{ 案例 (a,b) => a*b}。

You see here a limited version of pattern matching when initializing variables. Note that this works not only for tuples:

val a :: b = List(1,2,3)
println(a) //1
println(b) //List(2, 3)

This feature seems to be borrowed directly from Haskell, where you can use patterns for initialization as well:

let (a,b) = getTuple 
in a*b

As Haskell has no mutable data, there is no way to assign something.

In Scala you could do something like this, but I guess this was considered too confusing, or maybe too difficult to implement. You can always use a match expression as usual, and often you need just a case, e.g. List((1,2),(3,4)).map{ case (a,b) => a*b }.

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等风来 2024-11-10 01:38:15

我的怀疑是，如果变量名称元组左侧没有“var”或“val”，编译器会将元组视为元组。也就是说，您实际上是在尝试将值分配给 Tuple3 的实例而不是三个变量，这对编译器来说毫无意义。

顺便说一句，在您的示例中使用函数和各种数据类型是不相关的。这是一个更简单的例子：

scala> var ( i, j, k ) = ( 1, 2, 3 )
i: Int = 1
j: Int = 2
k: Int = 3

scala> ( i, j, k ) = ( 4, 5, 6 )
<console>:1: error: ';' expected but '=' found.
       ( i, j, k ) = ( 4, 5, 6 )
                   ^

My suspicion would be that without the "var" or "val" to the left of the tuple of variable names, the compiler treats the tuple as a tuple. That is, you're really trying to assign a value to an instance of Tuple3 and not to the three variables, and that makes no sense to the compiler.

Incidentally, using a function and various datatypes in your example isn't relevant. Here's a simpler example:

scala> var ( i, j, k ) = ( 1, 2, 3 )
i: Int = 1
j: Int = 2
k: Int = 3

scala> ( i, j, k ) = ( 4, 5, 6 )
<console>:1: error: ';' expected but '=' found.
       ( i, j, k ) = ( 4, 5, 6 )
                   ^

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