Concatenative styles are making the rounds lately including some interesting examples in Clojure and Haskell. Now I’m perpetually drawn to concatenative languages and their ideas, so I wanted to know what it would take to provide a minimally representative concatenative experience in Clojure.¹ As it turns out… not much:

(defn postfix [& e]
  (reduce #(if (fn? %2)
             (let [[l r & m] %]
               (cons (%2 r l) m))
             (cons %2 %)) [] e))

This is how it’s used:

(postfix 5 1 2 + 4 * + 3 -)
;;=> [14]

The way that it works is that it treats every non-function as a value and every function as a binary function taking two elements off of a “stack”. It then wraps the answers back up onto the “top” of the stack. The final answer is the stack itself.

This is in no way indicative of a fully blown representation of concatenative programming languages,² but it was fun and it highlights the stark simplicity of the model. This is my year to finally take concatenative languages seriously. I plan to study hard. I hope you’ll join me.

:F

if you want to talk concatenative languages or pesto5, then feel free to comment below or email me at the address at the top of my blog