In Swift 2.1 (running XCode 7.2), I am trying to have a Protocol with associated type conform to Equatable.
在Swift 2.1(运行XCode 7.2)中,我试图让一个具有相关类型的协议符合Equatable。
// (#1)
/**
A Node in a graph
*/
public protocol GraphNode : Equatable {
typealias Content : Equatable
/**
The content of the node.
E.g. in a graph of strings, this is a string
*/
var content: Content {get}
/// The list of neighbours of this Node in the graph
var children: [Self] {get}
}
As we could have non-homogeneous implementations of the protocol that define a different type for the associated type, I expect that I won't be able to define here (at the protocol level, not at the implementation level) an equality function:
由于我们可以为协议定义不同类型的协议的非同类实现,我希望我不能在这里定义(在协议级别,而不是在实现级别)一个相等的函数:
// (#2)
/// Won't compile, as expected
public func ==(lhs: GraphNode, rhs: GraphNode) {
return lhs.content == rhs.content
}
This is because I have no guarantee that lhs.Content is the same type as rhs.Content. However I was hoping I could specify it with some generic constraint, such as:
这是因为我无法保证lhs.Content与rhs.Content的类型相同。但是我希望我能用一些通用约束来指定它,例如:
// (#3)
/// Won't compile, why?
public func ==<Node1 : GraphNode, Node2 : GraphNode where Node1.Content == Node2.Content>(lhs: Node1, rhs: Node2)
{
return lhs.content == rhs.content // ERROR: Binary operator '==' cannot be applied to two 'Node1.Content' operands
}
In #3, we know that both lhs and rhs have the same type, and we know (from the specification of the associated type as Equatable) that Content is equatable. So why can't I compare them?
在#3中,我们知道lhs和rhs都具有相同的类型,并且我们知道(从相关类型的规范为Equatable)Content是等同的。那么为什么我不能比较它们呢?
1 个解决方案
#1
1
Add -> Bool. Just a bad error message. Sometimes writing function declaration across multiple lines doesn't make it more readable.
添加 - > Bool。只是一个错误的错误消息。有时在多行中编写函数声明并不会使其更具可读性。
public func ==<Node1 : GraphNode, Node2 : GraphNode where Node1.Content == Node2.Content>(lhs: Node1, rhs: Node2) -> Bool {
return (lhs.content == rhs.content)
}
#1
1
Add -> Bool. Just a bad error message. Sometimes writing function declaration across multiple lines doesn't make it more readable.
添加 - > Bool。只是一个错误的错误消息。有时在多行中编写函数声明并不会使其更具可读性。
public func ==<Node1 : GraphNode, Node2 : GraphNode where Node1.Content == Node2.Content>(lhs: Node1, rhs: Node2) -> Bool {
return (lhs.content == rhs.content)
}