Question

Clojure:

1:13 user=> (first (conj '(1 2 3) 4))
4
1:14 user=> (first (conj [1 2 3] 4))
1
; . . .
1:17 user=> (first (conj (seq [1 2 3]) 4))
4

I understand what is going on, but should this work differently?

Answer 1

Documentation for conj (from clojure.org):

conj[oin]. Returns a new collection with the xs
‘added’. (conj nil item) returns (item). The ‘addition’ may
happen at different ‘places’ depending on the concrete type.

It’s more efficient to “add” elements to the end of a vector, while it’s more efficient to do so at the beginning of lists. conj uses whatever is the most efficient for the data structure you give it.

In the examples you give, '(1 2 3) and (seq [1 2 3]) both implement ISeq (see documentation for seq?), while [1 2 3] doesn’t.

Clojure’s conj ultimately calls the cons method (not to be confused with the cons function – this method is internal clojure code) on the underlying data structure; for vectors (PersistentVector), cons adds elements to the end, while for lists they’re added to the front (the cons method for PersistentLists returns a new list with the new element as its head, and the existing list as its tail).