如何在Swift中返回序列? [英] How do I return a sequence in Swift?

问题描述

我正在尝试为本书中的Matrix示例编写扩展，稍作调整，使其具有通用性 .
我正在尝试编写一种名为getRow的方法，该方法在给定的行中返回一个值序列.

I'm trying to write an extension for the Matrix example from the book, slightly tweaked to be generic.
I'm trying to write a method called getRow that returns a sequence of values at the given row.

在C#中，我会这样写:

In C#, I would have written this:

IEnumerable<T> GetRow (int row)
{
    return Enumerable
        .Range (0, this.columns)
        .Select ((column) => this.grid[row, columns]);
}

或者

IEnumerable<T> GetRow (int row)
{
    for (var column = 0; column < this.columns; column++) {
        yield return this.grid[row, column];
    }
}

我不确定如何在Swift中执行此操作.

I'm not sure how to do this in Swift though.

Sequence似乎等同于IEnumerable<T>，但我不明白为什么它使用typealias而不是仅仅定义为Sequence<T>(

Sequence seems to be the equivalent to IEnumerable<T> but I don't understand why it uses typealias instead of just being defined as Sequence<T> (see also this). Defining a method that returns generic Sequence<T> did not work:

extension Matrix {
    // Cannot specialize non-generic type 'Sequence'
    func getRow<T>(index: Int) -> Sequence<T> {
        return map(0..self.columns, { self[index, $0] })
    }
}

然后我摆脱了<T>(但是它应该是通用的吗?):

Then I got rid of <T> (but how is it supposed to be generic?):

extension Matrix {
    func getRow(index: Int) -> Sequence {
        return map(0..self.columns, { self[index, $0] })
    }
}

此编译！但是我不能使用它:

This compiles! However I can't use it:

var row = grid.getRow(0)
// 'Sequence' does not conform to protocol '_Sequence_'
for i in row {
    println("\(i)")
}

如何正确键入map结果，以便可以在for..in循环中使用它?

How do I properly type map result so it can be consumed in a for..in loop?

有关此问题的更多信息:被视为怪异的关联类型

More on this issue: Associated Type Considered Weird

推荐答案

Joe Groff 建议包装结果在SequenceOf<T>中:

Joe Groff suggested to wrap the result in SequenceOf<T>:

extension Matrix {
    func getRow(index: Int) -> SequenceOf<T> {
        return SequenceOf(map(0..self.columns, { self[index, $0] }))
    }
}

确实可以，但是我们不得不将map结果包装到一个帮助程序类中，该类与我在C#中的用法不同.

Indeed, this works but we had to wrap map result into a helper class which differs from how I do it in C#.

我必须承认我还不明白为什么Sequence和Generator使用typealias并且不是通用协议(例如C#中的IEnumerable<T>).正在进行有关此区别的有趣讨论，因此我会留出一些链接以示好奇:

I have to admit I don't yet understand why Sequence and Generator use typealias and aren't generic protocols (like IEnumerable<T> in C#). There is an interesting ongoing discussion about this distinction so I'll leave a few links for a curious mind:

1. 被认为很奇怪的关联类型
2. 关联的类型与类型参数-前者的原因?
3. Scala中的抽象类型成员与通用类型参数
4. 泛型和协议

1. Associated Types Considered Weird
2. Associated types vs. type parameters - reason for the former?
3. Abstract Type Members versus Generic Type Parameters in Scala
4. Generics and protocols

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