Clojure Macros: ->> (Thread-Last)

- 2 mins

One of the most popular features and advantages of Clojure or any Lisp-based language are macros. Today I had the chance to use one as a part of a refactoring I was doing so I got a first-taste of a Clojure macro, in this case the ->> macro commonly-known as thread-last.

Definition: Threads the expr through the forms. Inserts x as the last item in the first form, making a list of it if it is not a list already. If there are more forms, inserts the first form as the last item in second form, etc.

If you’re like me and find definitions without any code examples incomplete, then worry not:

(defn winner? [board]
  (some true? (map #(all-equal-not-empty %) (lines board))))

This is the code I was refactoring today. Starting from the right and innermost call:

The above was refactored to the following:

(defn winner? [board]
  (->> (lines board)
       (map (partial all-equal-not-empty))
       (some true?)))

You can think of the above as “Take the result of (lines board) and feed it as a last argument to the next form. Then repeat the process.” So the next form in the code is (map (partial all-equal-not-empty)) so in this case map takes two arguments, the function which a collection will be mapped through and a collection. (lines board) is a collection and will step in as the second argument. The result of that will then go through the (some true?) in similar fashion.

Another part of the refactoring was partial which I will explain in another post. There’s a time and place for everything and I think the use of the ->> macro in this situation does make the code slightly more readable and clear.

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