Question

Let’s assume that we have an expensive computation expensive. If we consider that map produces a lazy seq, then does the following evaluate the function expensive for all elements of the mapped collection or only for the last one?

(last
  (map expensive '(1 2 3 4 5)))

I.e. does this evaluate expensive for all the values 1..5 or does it only evaluate (expensive 5)?

Answer 1

The whole collection will be evaluated. A simple test answers your question.

=> (defn exp [x]
     (println "ran")
     x)
=> (last
     (map exp '(1 2 3 4 5)))
ran
ran
ran
ran
ran
5