Threading Macros In Clojure 2: Electric Boogaloo

Oct 08, 2014

By Jamison Dance

More on Clojure's threading macros

Earlier I wrote about Clojure's humble threading macro. It turns out that like Transformers and ant nests there is more to threading macros in Clojure than meets the eye. Also, they are apparently called thrush operators.

->>, or Thread Last

-> threads an expression as the second item in an function call. In comparison, ->> threads the expression as the last item. Think of the extra angle bracket as a speed line showing how the arrow is going faster, so it flies in to the last position of the function call.

This is particularly helpful when working with sequences and functions that operate on sequences since they usually take a function as the second argument and the sequence as the last argument. For example, lets say we want to yell about what things we don't like to see in job postings.

(def bad-signs-in-job-postings ["facebook for" "rockstar" "guru" "ninja" "bro" "red bull" "crush" "disrupt"])
; actually disrupt is ok
(apply .toUpperCase (apply str (filter #((not (= "disrupt" %))) (shuffle bad-signs-in-job-postings))))

This looks a little tricky. Since apply, filter, and most of the functions that operate on seqs take the sequence in the last position, -> won't help us here. What is that whooshing sound you hear? It is ->>, whizzing in to the rescue! Let's rewrite it and see how it looks.

; actually disrupt is ok
(->> bad-signs-in-job-postings shuffle (filter #((not (= "disrupt" %)))) (apply .toUpperCase) (clojure.string/join " "))

We thread bad-signs-in-job-postings through shuffle, filter, apply, and join, with the result of each function call getting inserted in the last position of the next function call. Now we are yelling about dumb startup rhetoric! Neat!

as->, or Thread However I Want

What if you want to mix up where something gets threaded? You could nest -> and ->>, but that can get sticky icky quickly. Fortunately, Clojure 1.5 added as->, which lets you go wild with threading.

as-> takes an expression, a name for that expression, and a bunch of forms that you use that name in. In each form the name represents the result of evaluating the previous form. Prose is hard, let's go coding.

(as-> [1 2 3 4] $
  (seq $)
  (nth $ 0)
  (+ $ 5)
; 6

Coming up with a non-contrived example is left as an exercise to the reader.

Other Threading Macros

There are a few other specialized threading macros. cond-> and it's sibling cond->> allow you to branch before threading, and some-> and some->> will thread a result through as long as it is not nil. My brain is tired so I failed to write examples of using these, but it is helpful to know they are out there. Now go forth and thread!

Jamison Dance enjoys kittens, making computers smarter and the magic of children's laughter. He tweets at @jergason and GitHubs as jergason. He also podcasts on JavaScript Jabber and Soft Skills Engineering.