Question

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}  

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; } 

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’?

Answer 1

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.