Question

Clojure’s split-with function is quite handy, but has to traverse the leading part of the seq twice, as it is literally implemented as [(take-while pred coll) (drop-while pred coll)]. Still, it is fairly easy to write a (tail-recursive) version that traverses the leading part only once (put the leading part in an accumulating vector, etc.).

However, I would like to extract the first element of a list that satisfies a predicate and return the both the element, and the remaining list (i.e. (concat (take-while pred coll) (next (drop-while pred coll)))) — hopefully in a single pass. If I were using some imperative language, I would just traverse the list, holding onto the last cell, and, once I get the element to pop out, fiddle with the “next pointer” of the previous cell to reconstruct the modified list, but this seems out of question in a functional language.

So is there a way to do that efficiently in Clojure?

Answer 1

For split-with (and similar tasks where you want to produce two outputs from one input), you can have any two of

• Laziness
• Immutability
• Perfect efficiency.

For example, if you don’t want laziness (of the first “dropped” portion), you can get the other two by implementing a tail-recursive version as you suggest.

All this is not really applicable to your current question, since you only want one output sequence, and I recommend kotarak’s solution (or something else like it). However, I thought you might like an explanation for why Clojure’s built-in split-with traverses the input sequence twice.