What does Clojure function #(:jerry @%) means

Question

I'm very new in Clojure. I'm learning with help from Clojure Koans. I found an answer with code below:

(= ["Real Jerry" "Bizarro Jerry"]
       (do
         (dosync
          (ref-set the-world {})
          (alter the-world assoc :jerry "Real Jerry")
          (alter bizarro-world assoc :jerry "Bizarro Jerry")
          (vec (map #(:jerry @%) [the-world bizarro-world]))))))

from: https://github.com/viebel/clojure-koans/blob/master/src/koans/16_refs.clj#L42

It's pretty unfriendly for Google to search like "Clojure @%". So I get nothing from Internet.

How does it works for the function "#(:jerry @%)"?

And the code below is the answer from me, but it doesn't work.

(= ["Real Jerry" "Bizarro Jerry"]
       (do
         (dosync
          (ref-set the-world {})
          (alter the-world assoc :jerry "Real Jerry")
          (alter bizarro-world assoc :jerry "Bizarro Jerry")
          (vec (map (fn [x] (:jerry x)) [the-world bizarro-world]))
         )))

Answer 1

#( ...) is a reader macro for anonymous function where % means the first argument passed to the function. For example:

#(println %)

is equivalent to:

(fn [x] (println x))

@ is a reader macro for deref so again:

@some-variable

is the same as:

(deref some-variable)

and is used to dereference a current value from one of the ref types.

Thus #(:jerry @%) is an anonymous function which when applied to a ref (e.g. an atom) will deref its current value and use it as an argument to call :jerry keyword as a function with the value.

Answer 2

the-world and bizarro-world are "derefable", which means that you can use @ in front to get their value.

You are using an anonymous function, indicated by #( ). In an anonymous function, the percent sign % indicates the argument to the function.

So @% means, "dereference the argument to this function."

:jerry is a keyword used as a function, which gets the value associated with the key :jerry in the map.

For example:

(def coll [(ref {:jerry 21})
           (ref {:jerry 42})])
=> #'user/coll

(map #(:jerry @%) coll)
=> (21 42)