[英]Using a context bound in a class type parameter
I was under the impression that context bounds would work only on methods: 我的印象是上下文边界只适用于方法:
trait Target[T]
class Post {
def pinTo[T : Target](t:T)
}
apparently context bounds can be used in class
(and presumably trait
) too: 显然,上下文界限也可以在
class
上使用(也可能是trait
):
trait Target[T]
class Post[T:Target] {
def pintTo[T](t:T)
}
Now I'm confused as to how the evidence can be provided to Post
? 现在我对如何向
Post
提供证据感到困惑?
class Business
implicit object ev extends Target[Business] // is implicit necessary here ?
val p = new Post[Business] // ?? how do I provide ev ?
related to Modeling a binary relationship between two types 与建模两种类型之间的二元关系有关
The A: Foo
notation for context bounds is only a shortcut for asking for an implicit value parameter of type Foo[A]
. 上下文边界的
A: Foo
表示法只是要求Foo[A]
类型的隐式值参数的快捷方式。 Since traits do not have constructor value parameters, you can not use this with a trait: 由于traits没有构造函数值参数, 因此不能将其与特征一起使用:
trait Foo[A]
trait Bar[A: Foo] // "error: traits cannot have type parameters with context bounds..."
Whereas in classes it's possible: 而在课堂上它是可能的:
class Bar[A: Foo] {
def foo: Foo[A] = implicitly[Foo[A]]
}
Which is just a different way of writing 这只是一种不同的写作方式
class Bar[A](implicit foo: Foo[A])
You provide the evidence like you do in any other normal method call: 您提供的证据就像您在任何其他常规方法调用中所做的那样:
new Bar[Int]()(new Foo[Int] {}) // explicitly
Or: 要么:
implicit val iFoo = new Foo[Int] {}
new Bar[Int] // implicitly
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.