swift - 函数和泛型

Question

我想在 swift 中实现“天真的”排序。我正在这样做:

extension Array {
    func sort<Element: Comparable>(isOrderedBefore: (Element, Element) -> Bool) -> [Element] {

        var sortedArray : [Element] = []
        for x in self {
            for (index, value) in sortedArray.enumerate() {
                if isOrderedBefore(x, value) {

                }
            }
        }
        return sortedArray
    }
}

我遇到编译错误:“无法使用参数列表 '(Element, Element) 调用 isOrderedBefore

如果删除 Comparable 协议(protocol)，代码可以编译，但如果数组元素不具有可比性，我就无法实现排序功能。

此代码编译:

extension Array {
    func sort(isOrderedBefore: (Element, Element) -> Bool) -> [Element] {

        var sortedArray : [Element] = []
        for x in self {
            for (index, value) in sortedArray.enumerate() {
                if isOrderedBefore(x, value) {

                }
            }
        }
        return sortedArray
    }
}

我做错了什么？

Answer 1

a) Array 符合 CollectionType 并间接符合 SequenceType b) 作为 SequenceType 的一部分，有默认的排序实现

extension SequenceType where Self.Generator.Element : Comparable {
    /// Return an `Array` containing the sorted elements of `source`.
    ///
    /// The sorting algorithm is not stable (can change the relative order of
    /// elements that compare equal).
    ///
    /// - Requires: The less-than operator (`func <`) defined in
    ///   the `Comparable` conformance is a
    ///   [strict weak ordering](http://en.wikipedia.org/wiki/Strict_weak_order#Strict_weak_orderings)
    ///   over the elements in `self`.
    @warn_unused_result
    public func sort() -> [Self.Generator.Element]
}

和

extension SequenceType {
    /// Return an `Array` containing the sorted elements of `source`
    /// according to `isOrderedBefore`.
    ///
    /// The sorting algorithm is not stable (can change the relative order of
    /// elements for which `isOrderedBefore` does not establish an order).
    ///
    /// - Requires: `isOrderedBefore` is a
    ///   [strict weak ordering](http://en.wikipedia.org/wiki/Strict_weak_order#Strict_weak_orderings)
    ///   over the elements in `self`.
    @warn_unused_result
    public func sort(@noescape isOrderedBefore: (Self.Generator.Element, Self.Generator.Element) -> Bool) -> [Self.Generator.Element]
}

因此，如果默认实现对您来说不够(如果您对比较相等的元素的顺序有一些特殊要求)，您可以遵循相同的场景

extension SequenceType where Self.Generator.Element : Comparable {
    public func mySpecialSort() -> [Self.Generator.Element] {
      ... 
    }
}

和/或

extension SequenceType {
    public func mySpecialSort(@noescape isOrderedBefore: (Self.Generator.Element, Self.Generator.Element) -> Bool) -> [Self.Generator.Element] {
   ...
}