protocol DArray: Sequence where Element: Numeric {
var elements: [Element] { get set }
subscript(index: Int) -> Element { get set }
static func *(lhs: Self, rhs: Self) -> Self
}
struct Vector<Element: Numeric>: DArray {
var elements: [Element]
init(_ elements: [Element] = []) {
self.elements = elements
}
...
static func *<T: DArray>(lhs: Self, rhs: T) -> Self {
var v = lhs
var result: Self
for (i, element) in rhs.enumerated() {
let e = v[i]
let r = element * e
// Cannot convert value of type 'Element' (generic parameter of generic struct 'Vector') to expected argument type 'T.Element' (associated type of protocol 'Sequence')
}
return result
}
}
对于Numeric
协议,文档说:
数字协议为在标量值,例如整数和浮点数。您可以编写可对标准中的任何数字类型进行操作的通用方法库,使用数字协议作为一般约束。
因此,我选择了数字协议作为Element类型以及T.Element的通用约束。尽管e
和element
都符合Numeric
协议,但我无法将它们相乘(得到错误消息:无法将类型为'Element'的值(泛型结构'Vector'的泛型参数)转换为预期的参数类型“ T.Element”(协议“序列”的关联类型))。我该怎么办?
如@Sweeper所述,您只能将相同的Numeric
相乘。
所以您必须使用where
子句在函数中指定它:
static func * <T: DArray> (lhs: Vector<Element>, rhs: T) -> Vector<Element> where T.Element == Element {
let result = zip(lhs, rhs).map { lhs, rhs in
lhs * rhs
}
return Vector(result)
}
因为乘法是可交换的,所以将*
的输出类型定义为操作数的任何一种都没有意义。相反,您应该允许所有DArray
均使用其元素进行初始化。
protocol DArray: Sequence where Element: Numeric {
var elements: [Element] { get set }
init<Elements: Sequence>(_: Elements) where Elements.Element == Element
}
extension Vector {
init<Elements: Sequence>(_ elements: Elements) where Elements.Element == Element {
self.init( Array(elements) )
}
}
然后,像这样定义运算符:
extension DArray {
static func * <DArray1: DArray, Output: DArray>(
dArray0: Self, dArray1: DArray1
) -> Output
where DArray1.Element == Element, Output.Element == Element {
.init( zip(dArray0, dArray1).map(*) )
}
}
这样,您可以根据需要显式键入结果。
struct 🏹: DArray, IteratorProtocol {
mutating func next() -> Int? { nil }
var elements: [Element] = []
init<Elements: Sequence>(_ elements: Elements) where Elements.Element == Element { }
}
( Vector() * 🏹([]) ) as Vector
( Vector() * 🏹([]) ) as 🏹
( 🏹([]) * Vector() ) as Vector
( 🏹([]) * Vector() ) as 🏹
let vector: Vector = ( Vector() * 🏹([]) )
let 💘: 🏹 = ( 🏹([]) * Vector() )