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"))

Why can't I use .isDirectory as a first-class function in Clojure? Is there a better way to rewrite this code?

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

Answer 2

Java methods aren't clojure functions because you can't call a method on its own; you have to call a method on an object, and it has to be an object of the type that the method expects. In other words, in java, a method cannot be fully separated from its defining class (at least not efficiently).

An alternative to #(.foo %) would be (memfn foo), which hardly anyone uses anymore after #(...) was introduced.

Answer 3

You could have a look at the sourcecode of file-seq (which uses a tree-seq) to see how it works.

By the way: your code works perfectly well for me. I just have to use java.io.File instead of File in the REPL so it knows the Java class.

Answer 4

You've already been given the correct answers, but just to add a bit more Clojure idiomatic code, I'd also use

#(.foo %)

as Joost Diepenmaat did (yet I believed it might've been overlooked).

I would also suggest reading Listing files in a directory in Clojure.