added fnil to supply default values #257

Signed-off-by: Stuart Halloway <stu@thinkrelevance.com>

src/clj/clojure/core.clj

@@ -5669,6 +5669,29 @@
                         (cons x (keepi (inc idx) (rest s)))))))))]
        (keepi 0 coll))))
 
+(defn fnil
+  "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."
+  ([f x]
+   (fn
+     ([a] (f (if (nil? a) x a)))
+     ([a b] (f (if (nil? a) x a) b))
+     ([a b c] (f (if (nil? a) x a) b c))
+     ([a b c & ds] (apply f (if (nil? a) x a) b c ds))))
+  ([f x y]
+   (fn
+     ([a b] (f (if (nil? a) x a) (if (nil? b) y b)))
+     ([a b c] (f (if (nil? a) x a) (if (nil? b) y b) c))
+     ([a b c & ds] (apply f (if (nil? a) x a) (if (nil? b) y b) c ds))))
+  ([f x y z]
+   (fn
+     ([a b] (f (if (nil? a) x a) (if (nil? b) y b)))
+     ([a b c] (f (if (nil? a) x a) (if (nil? b) y b) (if (nil? c) z c)))
+     ([a b c & ds] (apply f (if (nil? a) x a) (if (nil? b) y b) (if (nil? c) z c) ds)))))
+
 (defn- ^{:dynamic true} assert-valid-fdecl
   "A good fdecl looks like (([a] ...) ([a b] ...)) near the end of defn."
   [fdecl]

test/clojure/test_clojure/other_functions.clj

@@ -50,6 +50,17 @@
        "bar" 'bar
        "quux" "quux"))
 
+(deftest test-fnil
+  (are [x y] (= x y)
+       ((fnil + 0) nil 42) 42
+       ((fnil conj []) nil 42) [42]
+       (reduce #(update-in %1 [%2] (fnil inc 0)) {} 
+               ["fun" "counting" "words" "fun"])
+       {"words" 1, "counting" 1, "fun" 2}
+       (reduce #(update-in %1 [(first %2)] (fnil conj []) (second %2)) {} 
+               [[:a 1] [:a 2] [:b 3]])
+       {:b [3], :a [1 2]}))
+
 ; time assert comment doc
 
 ; partial