[英]Scala val syntax: What does val myVal:{ def … } mean?
I am new to Scala and to funcprog. 我是Scala和funcprog的新手。
I have a piece of code (some of you might recognize it): 我有一段代码(你们中的一些人可能会认出来):
trait SwingApi {
type ValueChanged <: Event
val ValueChanged: {
def unapply(x: Event): Option[TextField]
}
...
}
where I do not underestand what val ValueChanged: {...} is. 在哪里我没有动摇和val ValueChanged:{...}是什么。
From this post I sort of learned that 从这篇文章我有点了解到
type ValueChanged <: Event
and 和
val ValueChanged: {
def unapply(x: Event): Option[TextField]
}
are two unrelated things because they are in different namespaces, etc, and type ValueChanged is an abstract subtype of Event . 是两个不相关的东西,因为它们在不同的命名空间等,而类型ValueChanged是Event的抽象子类型。
Good, then I try in a Scala worksheet: 好的,然后我在Scala工作表中尝试:
type myString <: String
val myString: {
def myfunc(x: String): String
}
and it shows me an error "only classes can have declared and undefined members"... Isn't it a similar construction? 它显示了一个错误“只有类可以声明和未定义的成员”......这不是一个类似的结构吗?
Finally, the questions are: 最后,问题是:
what is ValueChanged in val ValueChanged part of code? val ValueChanged代码中的ValueChanged是什么?
is it truely so unrelated to type ValueChanged <: Event 是否真的与TypeChanged <:Event类型无关
what does this syntax mean: 这个语法是什么意思:
val myVal:{def func{x:T}:T} val myVal:{def func {x:T}:T}
? ? What's the name of the value, its type and its actual value here?
这里的价值名称,类型和实际价值是多少?
Thanks! 谢谢!
{def unapply(x: Event): Option[TextField]}
is a structural Type, it means that it accepts any Object that has an unapply method with a Event as Parameter and Option[TextField] as return value. 是一个结构类型,它意味着它接受任何具有unapply方法的Object,其中Event为Parameter,Option [TextField]为返回值。 It is most often used similar to Duck typing, eg:
它通常类似于Duck类型,例如:
def foo(canQuack: {def quack(): Unit}) = {
canQuack.quack()
}
object Bar{
def quack(): Unit = print("quack")
}
object Baz{
def bark(): Unit = print("bark")
}
foo(Bar) //works
foo(Baz) //compile error
so 所以
type StructuralType = {def unapply(x: Event): Option[TextField]}
val ValueChanged: StructuralType
declares a val with the name ValueChanged and with the Type StructuralType, but assigns no value, that makes only sense in a trait or abstract class, thats why your example doesnt work. 声明一个名为ValueChanged并使用Type StructuralType的val,但是没有赋值,这只在特征或抽象类中有意义,这就是为什么你的例子不起作用的原因。
So what 所以呢
trait SwingApi {
...
val ValueChanged: {
def unapply(x: Event): Option[TextField]
}
...
}
means is that the trait SwingApi can only be applied to Objects/Classes that have a val with the name ValueChanged and whatever value is assigned to it, has an unapply method 意味着特征SwingApi只能应用于具有名称为ValueChanged的val的对象/类以及分配给它的任何值,具有unapply方法
trait SwingApi {
val ValueChanged: {
def unapply(x: Event): Option[TextField]
}
}
//works:
object Bar extends SwingApi{
val ValueChanged = {
def unapply(x: Event): Option[TextField] = None
}
}
//compile error:
object Baz extends SwingApi{
val ValueChanged = {
//wrong name
def foo(x: Event): Option[TextField] = None
}
}
//compile error:
object Baz2 extends SwingApi{
val ValueChanged = {
//wrong input/output type
def unapply(): Unit = {}
}
}
All code untested 所有代码未经测试
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