Jim Cheung

Professional Clojure

reading notes from Professional Clojure

Chapter 1 - Have a Beginner's Mind

In computer science, imperative programming is a programming paradigm that uses statements that change a program's state.

Value Oriented

Clojure promotes a style of programming commonly called "value-oriented programming."

(defn fib [n]
    (= n 0) 0
    (= n 1) 1
    :else (+ (fib (- n 1))
             (fib (- n 2)))))

You can rewrite a function to leverage memorization to see a signicant improvement in the execution time:

(def memoized-fib
  (memoize (fn [n]
               (= n 0) 0
               (= n 1) 1
               :else (+ (fib (- n 1))
                        (fib (- n 2)))))))

Thinking Recursively

A naive recursive defnition of a factorial in Clojure may look like the following:

(defn factorial [n]
  (if (= n 1)
    (* n (factorial (- n 1)))))

tail recursive style (using recur):

(defn factorial2 [n]
  (loop [count n acc 1]
    (if (zero? count)
      (recur (dec count) (* acc count)))))

trampoline and letfn:

(defn my-even? [n]
  (letfn [(e? [n]
            (if (= n 0)
              #(o? (dec n))))
          (o? [n]
            (if (= n 0)
              #(e? (dec n))))]
    (trampoline e? n)))

(defn my-odd? [n]
  (not (my-even? n)))

higher order functions

(defn wrapInTransaction [f]

(wrapInTransaction #(
                       (add order user)
                       (addLineItemsFrom cart orderKey))))

Chapter 2 - Rapid Feedback Cycles with Clojure

Chapter 3 - Web Services

Chapter 4 - Testing

Chapter 5 - Reactive Web Pages in ClojureScript

Chapter 6 - The Datomic Database

Chapter 7 - Performance