(defn seq-trial
[]
(map #(do (println "hello " %) (inc %)) (range 10)))
(take 3 (seq-trial))
The code snippt above when evaluated prints out the following -
(hello 0 hello 1 hello 2 hello 3 hello 4 hello 5 hello 6 hello 7 hello 8 hello 9 1 2 3)
Because map returns a lazy sequence I expected this to print only -
(hello 0 hello 1 hello 2 1 2 3)
Why is the entire list evaluated here?
This is because of a performance optimization called chunking. Essentially the sequence is realised in groups of n items called chunks. This means you need to take care with any side effects in your mapping function. The end result is correct tho, you still get a sequence of the correct length returned
The default chunk size is 32, so if you increase your range to a value larger than that you will understand what's going on a bit better:
user> (defn seq-trial
[]
(map #(do (println "hello " %) (inc %)) (range 100)))
user> (take 3 (seq-trial))
hello 0 ; 32 item 'chunk' realized...
hello 1
...
hello 30
hello 31
(1 2 3) ; the expected returned value
If you need to avoid chunking there are options available