alexpw

(defn group-by-with
  "A group-by factory that accepts a combiner-fn to control how the values
  of the grouping fn are collected."
  [combiner-fn]
  (fn [f coll]
    (persistent!
     (reduce
      (fn [ret x]
        (let [k (f x)]
          (assoc! ret k (combiner-fn (get ret k) x))))

alexpw

(comment
(defproject project "0.0.1-SNAPSHOT"
  :description "project: analysis framework"
  :dependencies [[org.clojure/clojure "1.4.0"]
                 [cheshire "5.0.0"]
                 [cascalog "1.10.0"]
                 [cascalog-math "0.1.0"]
                 [cascading.avro/avro-scheme "2.1.0"]
              ]
  :dev-dependencies [[org.apache.hadoop/hadoop-core "0.20.2-dev"]]

alexpw

(defaggregateop variance
  "An implementation of the Welford/Knuth online algorithm:
   http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#On-line_algorithm"
  ([] [0 0 0])
  ([[m2 mean n] val]
    (let [n (inc n)
          delta (- val mean)
          mean (+ mean (div delta n))
          m2 (+ m2 (* delta delta))]
      [m2 mean n]))

alexpw

user=> (defn foo [{:keys [x y] :or {x 10}}] (println "x:" x "y:" y)) 
#'user/foo
user=> (foo {:y 1})                                                  
x: 10 y: 1
nil
user=> (foo {:y 1 :x 2})
x: 2 y: 1
nil

alexpw

;; Approach 1, verbose

(defmacro try-catch [[lvl-fn fn] body] 
  (list 'try body 
    (list 'catch 'Exception 'e
      (list lvl-fn 'e
        (list ':name (list 'meta '#'fn))))))

(macroexpand '(try-catch [info foo] (reduce + 0 (range 5))))

alexpw

(defn my-math
  "Example using optional args through named keys."
  [nums & {:keys [operation]
           :or {operation +}}]
  (apply operation nums))

user=> (my-math '(1 2 3))
6
user=> (my-math '(1 2 3) :operation *)
6