Through the labyrinth of the internet I somehow ended up watching Rich Hickey, the creator of Clojure. Now and then the stars align and I navigate Youtube, which I found to be badly sorted and organized.

It’s a great talk and it made me want to try out Clojure. My goal was to learn Swift, but, …, procrastination happened? I did a dozen or so exercises on Hackerrank.

One of the exercises was to reverse a list, without using the built-in function. Here it started to be weird.

My first thought was to use the reduce function where you walk through the list and built up a new result. conj[oin] is used to return a new collection with the element added.

Let’s skip reading the documentation completely of course :)

Doesn’t work:

(fn [list]
(reduce conj [] lst)
)


Works:

(fn [list]
(reduce conj () lst)
)


There is a difference based on the type () is a list, [] is a vector. And, as is stated clearly in the documentation conj works differently for each datatype:

• For lists it adds at the end, the underlying implementation is a singly-linked list. So, makes sense.
• For vectors it adds at the beginning

Probably the best option is to use sequences and use cons. But how that would exactly work…a quick try in the editor of Hackerrank just gave me stack traces that my function definition was wrong. Copy-pasting an example from the Clojure docs and the problem persisted.