在 C# 中编写 F# 递归文件夹访问者 - seq 与 IEnumerable [英] Writing the F# recursive folder visitor in C# - seq vs IEnumerable

问题描述

我经常在 F# 中使用这个递归的访问者"

I often use this recursive 'visitor' in F#

let rec visitor dir filter= 
    seq { yield! Directory.GetFiles(dir, filter)
          for subdir in Directory.GetDirectories(dir) do yield! visitor subdir filter} 

最近我开始致力于在 C# 中实现一些 F# 功能，并且我正在尝试将其复制为 IEnumerable，但我很难做到:

Recently I've started working on implementing some F# functionality in C#, and I'm trying to reproduce this as IEnumerable, but I'm having difficulty getting any further than this:

static IEnumerable<string> Visitor(string root, string filter)
{
    foreach (var file in Directory.GetFiles(root, filter))
        yield return file;
    foreach (var subdir in Directory.GetDirectories(root))
        foreach (var file in Visitor(subdir, filter))
            yield return file;
}

我不明白的是为什么我必须在 C# 版本中为递归执行双重 foreach，而在 F# 中则不然... seq {} 是否隐式执行了concat"?

What I don't understand is why I have to do a double foreach in the C# version for the recursion, but not in F#... Does the seq {} implicitly do a 'concat'?

推荐答案

yield! 执行展平"操作，因此它将您传递给它的序列集成到外部序列中，隐式执行 foreach 在序列的每个元素上，yield 在每个元素上.

yield! does a 'flatten' operation, so it integrates the sequence you passed it into the outer sequence, implicitly performing a foreach over each element of the sequence and yield on each one.

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