Working with apply

apply.clj

(ns functions.apply)

;; We can use apply with print because print takes the (& more)
;; parameter.  So we can pass any number of args to messenger
;; and then apply will explode the sequence generated by (& who) in
;; the argument list into the print method
(defn messenger "Fooling with apply function" [greeting & who]
  (println who)
  (apply print greeting who))

(messenger "Hello" "world" "class" "\n")
(messenger "Hello" "Adam" "and" "Ginger" "\n")

;; Without the apply function we print the vector world class
(print "Hello" ["World" "class"])
;; no more vector
(apply print "Hello" ["World" "class"])

(defn add-from-100
  "Testing out apply"
  ;; Takes a sequence and then 'explodes' the sequence to the same function
  ;; Trick to using apply is it must send to a function that takes a (rest)
  ;; param.
  ([xs]
   (apply add-from-100 xs))
  ;; Here we get the first val and then get the rest of the params.
  ;; Since + takes a rest param (& more) as one of its args we can use
  ;; apply to sum up the rest of our args
  ( [x & xs]
   (apply + 100 xs)))

(add-from-100 1 2 10 100 1000)
(add-from-100 [1 3 10 100 1000])

(max 4 2 3 5 7 1 2 99)
;; Another example is the max function.  Lets say we got a vector of values from
;; some data source.  If we just use the max function it will output the vector
(max [4 2 3 5 7 1 2 99])
;; if we use apply, it will "explode" those values into something max can process
;; this works because if you look at (doc max) you will see one of the argument lists
;; looks like [x y & more]...essentially x = 4, y = 2, and more is the other data 
(apply max [4 2 3 5 7 1 2 99])