A Clojure map can occupy the callable position in a list. Its first argument is the key to look up; an optional second argument supplies a value when that key is absent. section 8.2.2
({:a 1 :b 2} :b) ; #1
;; 2
({:a 1 :b 2} :c "Not found") ; #2
;; "Not found"
The first call returns the value for :b. The second returns "Not found" because :c is absent. section 8.2.2
This is a concise alternative to get. Hash maps implement clojure.lang.IFn for invocation, and both forms ultimately use the clojure.lang.ILookup valAt method. Prefer get when its handling of nil targets or Java maps matters. Keywords can also be invoked, but reverse the argument order: the keyword receives the map. section 8.2.2
