How to get an Fn which calls a Java method?

Question

I\'m learning Clojure. I wrote this code to recursively walk a directory.

(tree-seq #(.isDirectory %1) #(.listFiles %1) (File. \"/my-directory\"))
         


        

Answer 1

Joost is spot on about Java methods not being first class functions.

As an approach to dealing with this, I usually like to wrap Java functions in a Clojure function (or find a library that does this already), then it is easy to use them in an idiomatic first-class way:

(defn directory? [^java.io.File file]
  (.isDirectory file))

(defn list-files [^java.io.File file]
  (.listFiles %1))

(tree-seq directory? list-files (File. "/my-directory"))

This is a few more lines of code, but has the following advantages:

• You can add type hints in your functions to avoid reflection (as above)
• The final code is cleaner and more idiomatic
• You have abstracted away from the underlying Java interop