Clojure - an introduction (and some CFML)

Clojure - an introduction Kai Koenig CFCAMP Munich 2012*and some CFML

Kai KoenigWorking with CFML since 1999Developing for mobile since 2003Recovering Flex developer being sucked into JS deeper and deeperRecently rediscovered Functional Programmingbloginblack.de2ddu.comtwitter.com/agentK

Agenda

1. Why Functional Programming?2. Why Clojure?3. Some demos and (probably failing :-) live coding inthe Clojure REPL4. A quick look at Clojure and CFML integration

Why FunctionalProgramming?

Who of you has atraditional, old-school CompSci or Maths degree?

The ones among you who answered with “yes”probably have dabbled with either of:MLSchemeHaskellLispAt some point during their degree.

I did. Haskell and ML ** Kind of confused me, I couldn’t see the point and we had Java anyway - which wasmuch more awesome & could do OO & was the hipster language of the day ** &you could write AWT and Swing apps and just because.** The day was 1996; Java was at version 1.0.2; and I was in my ﬁrst year of myCompSci and Maths degree

What is it?A programming paradigm based on functions as ﬁrst-class citizens and a theoretical concept called “LambaCalculus”.Depending on the FP language you’re looking at,features might vary, but there are a few commonthemes in functional programming.

First-class and higher- order functions

Function that can take other functions as arguments orreturn them as results (e.g. d/dx - differential operatorin calculus to return the derivative of a function).Higher-order vs. ﬁrst-class: just a subtle difference -previous is a mathematical concept, latter is a CompSciterm.

Pure functions

Functions without I/O and memory side effects.1. result is constant with respect to the parameter list2. function call can be performed in parallel, they can’tinterfere with each other; thread safety3. compiler has much more freedom in the evaluationstrategy of expressions provided the whole language isside effect-free.

Recursion

In functional languages, iteration (loops) are prettymuch exclusively done via recursion.Recursion functions invoke themselves all over again.Intermediate state is kept on a stack.

Evaluation strategy (eager vs. lazy)Type system (untyped lambda calculus vs. type lambdacalculus)

But hang on. The world is full of “state”.And side-effects.I mean - WTF?

State can (and has to) be simulated in FP.Different languages use different concepts that still keepthe functional paradigm pure.Monads (Haskell)State passing style (A bunch of other FP languages)Managed references (Clojure)

How does FP “feel”?

Fibonacciiterative in Java

public static long itFibN(int n){ if (n < 2) return n; long ans = 0; long n1 = 0; long n2 = 1; for(n--; n > 0; n--) { ans = n1 + n2; n1 = n2; n2 = ans; } return ans;}

Fibonaccirecursive in Haskell

-- describe an infinite list based on the recurrencerelation for Fibonacci numbersfibRecurrence first second = first : fibRecurrencesecond (first + second)-- describe fibonacci list as fibRecurrence with initialvalues 0 and 1fibonacci = fibRecurrence 0 1-- describe action to print the 10th element of thefibonacci listmain = print (fibonacci !! 10)

Fibonaccirecursive in Clojure

(def fib (lazy-cat [0 1] (map + fib (rest fib))))execute:(take 10 fib)

Moore’s law failedNeed for concurrency

State in imperative and OO languages

OOP encapsulates stateChanges overwrite stateState has multiple observersConcurrency requires lockingLocking is complicated

Let’s look at Clojure

Clojure: dynamically typed, JVM-based Lisp dialect (by Rich Hickey in 2007)

But... why?

ReadEvalPrintLoop

Some Clojure features

Code is data

Literals

Java Interop

List,Vector, Map and Set

Life without for-loops

What is it good for?

Big data & AnalyticsPattern MatchingHeavy concurrency, scalable systemsHard problems (often related to formal math oralgorithmic work)

Clojure and CFML

CFML is Java, right?

There is cfmljure!https://github.com/seancorﬁeld/cfmljure

Where and how would you use it?

One idea: Model in MVC appsWhy? Because Clojure allows you to express businesslogic and model data extremely efﬁciently.Or: when you need to be able to run certain code in ahighly parallelised environment.

Anyway - it’s fun :)

Get in touchTwitter: @AgentKBlog: http://bloginblack.dePodcast (2 Developers Down Under): http://2ddu.comAbout me: http://about.me/agentk

Clojure - an introduction (and some CFML)

by Kai Koenig on Oct 17, 2012

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