Creating a Lazy Sequence of Directory Descendants in Clojure

Update: it turns out Clojure has a built-in function file-seq that does (almost) exactly this. The difference is that you get a lazy sequence of File objects, not paths. Hooray for "batteries included"!

I was playing around with Clojure this morning, attempting to write a little photo viewer application. As part of this, I wound up writing a function that produces a lazy sequence of all the files that are descendants of a given directory. I was pretty happy with how it turned out, so I thought I’d share it. Here’s the code in its entirety: 

(import [java.io File])

(defn dir-descendants [dir]
  (let [children (.listFiles (File. dir))]
    (lazy-cat 
     (map (memfn getPath) (filter (memfn isFile) children)) 
     (mapcat dir-descendants 
      (map (memfn getPath) (filter (memfn isDirectory) children))))))

The key bit here is the lazy-cat, which returns a lazy sequence – it doesn’t evaluate the second expression (in which I walk into the children of the current directory) until someone asks for the next element in the sequence. In other words, even though this function looks recursive, it’s not. Because lazy-cat returns right away, the “nested” call to  dir-descendants happens after the stack has been popped. This is totally awesome, because it means I can call dir-descendants and it’ll return right away. So I can start displaying pictures without having to wait for the entire directory tree to be enumerated. 

Two things struck me about writing this code. The first is that it’s really, really compact. It would be even smaller if it weren’t for the calls to memfn, which is just there so I can use a Java method as if it were a Clojure function. The second is that it was a bit of a mind-bender to write, although having written it I find it fairly easy to read. But I suspect both of those things would also be true if I attempted to write the equivalent C# (including the laziness, probably via a “yield return” implementation of IEnumerable somewhere), although it would almost certainly be at least somewhat more verbose.