Question

I am learning the concept of sequence and nil in Clojure. This was the result of a small experimentation.

1:6 user=> (first '())
nil
1:7 user=> (rest '())
()
1:8 user=> (first (rest '()))
nil

Does this mean that ‘() is actually a sequence of nils?

Answer 1

If you want to test whether the “rest” of a collection is empty, use next.

user> (next '(foo bar))
(bar)
user> (next '())
nil
user> (doc next)
-------------------------
clojure.core/next
([coll])
  Returns a seq of the items after the first. Calls seq on its
  argument.  If there are no more items, returns nil.

“nil-punning” (treating an empty collection/seq and nil as the same thing) was removed last year in favor of fully-lazy sequences. See here for a discussion leading up to this change.