->

The thread-first macro "threads" an expression through several forms as the second item in a list.

Inserts x as the second item in the first form, making a list of it if it is not a list already. If there are more forms, inserts the first form as the second item in second form, etc.

(-> x
  (a b c)
  d
  (x y z))

(x (d (a x b c)) y z)

The first is arguably a bit more cumbersome to read than the second:

(first (.split (.replace (.toUpperCase "a b c d") "A" "X") " "))
;;=> "X"

(-> "a b c d"
    .toUpperCase
    (.replace "A" "X")
    (.split " ")
    first)
;;=> "X"

It can also be useful for pulling values out of deeply-nested data structures:

(def person
  {:name "Mark Volkmann"
   :address {:street "644 Glen Summit"
             :city "St. Charles"
             :state "Missouri"
             :zip 63304}
   :employer {:name "Object Computing, Inc."
              :address {:street "12140 Woodcrest Dr."
                        :city "Creve Coeur"
                        :state "Missouri"
                        :zip 63141}}})

(-> person :employer :address :city)
;;=> "Creve Coeur"

Same as above, but with more nesting:

(:city (:address (:employer person)))
;;=> "Creve Coeur"

It can also help with arithmetic:

(def c 5)
(-> c (+ 3) (/ 2) (- 1))
;;=> 3

Same as above, but with more nesting:

(- (/ (+ c 3) 2) 1)
;;=> 3