Get into Functional Programming with Clojureby John Stevenson @jr0cket

clojure.practical.li

Why Functional Programmingit's not just because it's really fun...

OO Softwarein actionJosh Smith - Tweet

The Complexity Iceberg

- @krisajenkins

● complexity is very dangerous when hidden

● You can't know what a function does for certain if it has side effects

Side Effects

Pure FunctionsThe results of the function are purely determined by its initial output and its own code

- no external influence, a function only uses local values- referential transparency (the function can be replaced by its value)

Impure Functions - side causesThe results of the function are purely determined by its initial output and its own code

- behaviour externally influenced and non-deterministic

Eliminating Side Effects

Functional programming is about eliminating side effects where you can, controlling them where you can't - @krisajenkins

The features in Functional Programming come from a desire to reduce side effects

ClojureGeneral purpose language hosted on JVM, JavaScript & CLR

Clojure / ClojureScript

A hosted language with simple interoperability with the host language

- (java.Util.Date.)- (js/alert “Client side apps are easier in Clojure”)

Persistent Data Structures

(list 1 2 3 4 5) ‘(“fish” “chip” 42)

(vec ‘(1 2 3 4)) [1 2 3 4]

{:key “value} {:name “John” :skill “talking in full rooms”}

(set ‘(1 2 3 4 4)) #{1 2 3 4}

Persistent Data Structures

Persistent Data Structures

Higher Order FunctionsFunctions always return a value & can be used as an argument to another function

- Chain functions over a data structure

Composing functions togetherExample: current value of the Clojure project from the configuration file

- `slurp` in the project file, convert into a string and return the value at index 2

RecursionProcess a collection of values by feeding the remaining elements back to the function

- the sum function has different behaviour depending on if passed 1 or 2 arguments

Recursion - tail call optomisationProtect the heap space from blowing by using the recur function

Lazy EvaluationOnly return a value when necessary

● maintain precision● optomise evaluation

Sequence / List Comprehension

Sequence / List Comprehension

Iterating through a range of generated values to create a list of 2 value vectors

Concurrency is Easier

Concurrency is much easier to write and reason about because of

- Immutability by default- Persistent Data Structures- values are immutable- functional isolation & pure functions- state changes managed atomically (software transactional memory)- core.async library allows you to write asynchronous code as easily as sequential

code

Safe State changes

Changing state safely by not changing it

● Persistent data structures● Local bindings

Changing state safely by changing it atomically

● Software Transactional Memory (STM)○ Gives an mechanism like an in-memory atomic database that manages mutable state changes

under the covers

● Atoms● core.async

Concurrency syntax - atoms

An online card game has players that can join and have their winnings tracked

Concurrency syntax - atoms

The join-game function adds players to the atom by their name, but only up to 2 players

Concurrency syntax - refs for sync updatesThe join-game-safely adds players to the ref and alters their account & game account

Putting it all togetherLet's find all the most common words used in a popular Science Fiction novel

ClojureGeneral purpose language hosted on JVM, JavaScript & CLR

Tools to learn Clojureinspire them & build up their motivation

Clojure support in many different tools

Leiningen - Clojure powered build automation

LightTable - Instarepl

Emacs & Spacemacs

Figwheel (flappy birds example)

Examples, examples, exampleswe learn by example...

Over 20 Books on Clojure...

Where to start with Clojure will be different...

Example:

I typically suggested BraveClojure.com as a starting point, however many people prefer LivingClojure or ClojureScript Unraveled...

Help people understand the relevance of a book and if it's the right thing for them at that time.

Clojure.org & ClojureDocs.org

Github

Clojure-through-code Git repository

http://practical.li/clojure-webapps

Testing your Clojure skills...

Clojurian Community in Person

Probably the most active language-specific developer communities in London

Learning by teaching others

I really started thinking in Clojure when I started talking to & teaching others

- Coding dojos- talks on Clojure (starting with the basics, showing the art of the possible)- moving on to running conferences- workshops at hack days

Overtone live performance - MetaX

Overtone live performance - MetaX

Take your own journey into Clojure

Thank you

@jr0cketjr0cket.co.uk