fnil

fnil.clj

-------------------------
;clojure.core/fnil
;([f x] [f x y] [f x y z])
;  Takes a function f, and returns a function that calls f, replacing
;  a nil first argument to f with the supplied value x. Higher arity
;  versions can replace arguments in the second and third
;  positions (y, z). Note that the function f can take any number of
;  arguments, not just the one(s) being nil-patched.

(+ nil 1)
;java.lang.NullPointerException (NO_SOURCE_FILE:0)
(def new+ (fnil + 0))
(new+ nil 1)

;EX-1
(def current-score {})                          
(defn update-score [current {:keys [country player score]}]
         (update-in current [country player] + score))

(update-score current-score {:player "Paul Casey" :country :England :score -1})
;java.lang.NullPointerException (NO_SOURCE_FILE:0)

;FIX1
(defn update-score [current {:keys [country player score]}]                    
         (update-in current [country player] #(+ (or %1 0) score)))         
(update-score current-score {:player "Paul Casey" :country :England :score -1})
;=> {:England {"Paul Casey" -1}}

;FIX using fnil
(defn update-score [current {:keys [country player score]}]                    
         (update-in current [country player] (fnil + 0) score))                
(update-score current-score {:player "Paul Casey" :country :England :score -1})
;=> {:England {"Paul Casey" -1}}