Threading Macros

Introduction#

Also known as arrow macros, threading macros convert nested function calls into a linear flow of function calls.

Thread Last (->>)

This macro gives the output of a given line as the last argument of the next line function call. For e.g.

(prn (str (+ 2 3)))

is same as

(->> 2
    (+ 3)
    (str)
    (prn))

Thread First (->)

This macro gives the output of a given line as the first argument of the next line function call. For e.g.

 (rename-keys (assoc {:a 1} :b 1) {:b :new-b}))

Can’t understand anything, right? Lets try again, with ->

(-> {:a 1}
    (assoc :b 1)                ;;(assoc map key val)
    (rename-keys {:b :new-b}))  ;;(rename-keys map key-newkey-map)

Thread as (as->)

This is a more flexible alternative to thread first or thread last. It can be inserted anywhere in the list of parameters of the function.

(as-> [1 2] x
      (map #(+ 1 %) x)
      (if (> (count x) 2) "Large" "Small"))