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(def fib-var-obj 
  (fn [x]
    (if (< x 2)
      1
      (+ (fib-var-obj (- x 1))
         (fib-var-obj (- x 2))))))

(defn fib-lex-obj [x]
  (if (< x 2)
    1

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(defmacro <- [& body]
  `(-> ~(last body) ~@(butlast body)))

(deftest <--test
  (is (= [2 3]
         (-> {1 1}
             (assoc 3 4)
             (update-in [1] inc)
             (->> (map-vals dec)
                  (map-keys inc)

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;; Take a map {:xs xs} and return a map
;; of simple univariate statistics of xs
(defn stats [{:keys [xs]}]
  (let [n  (count xs)
        m  (/ (sum identity xs) n)
        m2 (/ (sum #(* % %) xs) n) 
        v  (- m2 (* m m))]
    {:n n   ; count   
     :m m   ; mean 
     :m2 m2 ; mean square

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(def almost-stats-graph
  {:n  (fn [xs]   (count xs))
   :m  (fn [xs n] (/ (sum identity xs) n))
   :m2 (fn [xs n] (/ (sum #(* % %) xs) n))
   :v  (fn [m m2] (- m2 (* m m)))})

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(def stats-graph
  {:n  (fnk [xs]   (count xs))
   :m  (fnk [xs n] (/ (sum identity xs) n))
   :m2 (fnk [xs n] (/ (sum #(* % %) xs) n))
   :v  (fnk [m m2] (- m2 (* m m)))})

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(defnk foo [x y [s 1]]
  (+ x (* y s)))

(foo {:x 2 :y 3 :s 2})
;; ==> 8

(foo {:x 2 :y 3})
;; ==> 5

(foo {:x 2})

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;; Functionally identical to (defn stats ...) above
(def stats (graph/eager-compile stats-graph)) 

(stats {:xs [1 2 3 6]})
; ==> {:n 4     
;      :m 3 
;      :m2 12.5 
;      :v 3.5) 

;; Result is error checked

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(def lazy-stats (graph/lazy-compile stats-graph))

(:m (lazy-stats {:xs [1 2 3 6]}))
; ==> 3
; WIN: :m2 and :v are not computed.


(def par-stats (graph/parallel-compile stats-graph))

(:v (par-stats {:xs [1 2 3 6]}))

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(defn observe-graph [g record-node-time!]
  (into {}
   (for [[k f] g]
    [k
     (with-meta
       (fn [m]
         (let [t0 (System/nanoTime)
               v (f m)
               t1 (System/nanoTime)]
           (record-node-time! k (- t1 t0))

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(defnk foo [x y [s 1]]
  (+ x (* y s)))

(foo {:x 2 :y 3 :s 2})
; ==> 8

;; 's' defaults to 1 
(foo {:x 2 :y 3})
; ==> 5