Should I prefer nil to empty collections?

Wed, 14 Aug 2019 09:12:35 +0000

This is a question about intersection of 3 different clojure aspects:

Collection functions treat nil as empty collection. Functions like filter, map, and even assoc happily accept nil for coll.
if and derived macros (when, and, or etc.) treat nil as falsey value.
empty? coerces it's input to seq, which leads to unnecessary allocations, and it's idiomatic to check for not-emptyness using seq instead of (not (empty? xs)). This is a common source of confusion, because (not (empty? xs))'s intent feels more clear then seq

With that said, I feel like consciously representing every empty collection in application as nil might be useful, because this:

(let [xs (get-non-empty-coll-or-nil)]
  (if xs
    (do-stuff-with xs)
    (do-nothing)))

...is more performant and clear than this:

(let [xs (get-possibly-empty-coll)]
  (if (seq xs)
    (do-stuff-with xs)
    (do-nothing)))

Two downsides I see to this approach:

having to use (fnil conj []) or (fnil conj #{}) instead of conj to ensure collections are vectors/sets, because conj-ing to nil creates a list, which I personally almost never use.
having to run all incoming collections on the boundaries of a system through not-empty.

What do you think?

Clojure Q&A - Recent questions tagged empty?