Why does clojure.core/rest output a list when input is a vector?

Question

Why does clojure.core/rest output a list when input is a vector?

This creates an unexpected effect:

(conj [1 2 3] 4)
; => [1 2 3 4]

(conj (rest [1 2 3]) 4)
; => (4 2 3)

I know that "it calls seq on its argument" from the docs which creates this effect. I don't understand why this is the desired effect. As a naïve user, I would expect (rest [1 2 3]) to behave like (subvec [1 2 3] 1). I know I could just use subvec for my use case. For the sake of learning, I would like to understand the rationale of rest, and use cases where outputting a list is desirable (even when the input is a vector).

Answer 1

The output of rest is NOT a list, but a seq, which is an even lower level abstraction. From the official documentation for rest:

Returns a possibly empty seq of the items after the first. Calls seq on its argument.

The confusion arises from the fact that both are printed between parens, but if you look closely, they are different:

user=> (list? (rest [1 2 3]))
false

user=> (seq? (rest [1 2 3]))
true

How it's a seq different from a list? seqs are implemented with an Interface that requires implementing first, rest and cons, but details are up to the collection implementation. For instance, vectors use their own implementation:

user=> (class (rest [1 2 3]))
clojure.lang.PersistentVector$ChunkedSeq

user=> (class (rest '(1 2 3)))
clojure.lang.PersistentList

List are an implementation that at least extends a basic Seq interface, and builds on top. For instance, clojure.lang.PersistentList implements the Counted interface which requires a constant-time version of count.

For a detailed description of the differences between Seqs and Lists, check these links:

• Differences between a seq and a list
• https://clojure.org/reference/sequences