Mar 9, 2011

Yesterday I demonstrated how to transform a mundane recursive function into a tail recursive function. However, I received some flak for that post because there are better ways to implement run-length encoding with Clojure. While certainly true, I think some of my critics1 missed the point of the exercise. ;-)

Having said that; their points are valid because in Clojure (and likely most, if not all functional languages) recursion should be seen as a low-level2 operation and avoided if at all possible. The better path is to take a survey of the available higher-order functions and plug them together to create new functions. We go through great pains to stress this in Joy of Clojure, so I will not retrace well-trodden ground. Instead, observe the following:

(def S '[a a a a b c c a a d e e e e]) (def pack (partial partition-by identity)) (pack S) ;;=> ((a a a a) (b) (c c) (a a) (d) (e e e e)) (def rle (comp (partial map (juxt count first)) pack)) (rle S) ;=> ([4 a] [1 b] [2 c] [2 a] [1 d] [4 e]) (def rld (partial mapcat (partial apply repeat))) (-> S rle rld) ;;=> (a a a a b c c a a d e e e e)

As a nice bonus, the definitions3 above are point-free.

:f