Why Clojure?

Why Clojure?

Why was it created?

What do we mean with complexity?

How does clojure makes things more simple?

When was Clojure

● 2007: introduction by Rich Hickey

● 2009: version 1.0

....

● 2013: version 1.5

What is Clojure

● A Lisp

● for Functional Programming

● Running on the JVM, CLR and browsers

● Designed for concurrency

A Lisp

Many concepts of Lisps are taken over by other languages:● Conditionals● First-class functions● Recursion● Garbage collection● Programs made up out of expressions

Functional Programming

● Provides a way out of the multi-core conundrum

● Makes programs simpler by decomplecting them.

Running on the JVM, CLR, browser

● JVM / CLR have huge benefits:○ Libraries○ Memory and resource management○ Efficient and robust

● JavaScript has a huge reach:○ It's the most widely deployed language○ Through ClojureScript, we can target everywhere

JavaScript runs.● Clojure is symbiotic with its host platform.

Designed for Concurrency

● Machines are not getting faster, they just have more cores.

● Shared access to mutable state makes multi-core programming a nightmare.

● Immutability makes many of the problems go away.

● Separating identity and state can solve all the other problems.

Why is Clojure

● Reducing complexity

● to be easy? not exactly...

What do we mean with

● Complex

● Simple

● Hard

● Easy

Complexity

To Complectbe interwoven or interconnected

Complex

Consisting of many different and connected parts.

Complexity

Complex

Simple

How I/we made things complex

● Teamwork ● Robots

○ Explore unknown maze

○ Detect barcodes --> action

○ Travel over seesaw

● How to test?○ We couldn't

● Unmanageable complexity

Complexity

Simple Easy

Complexity

HardComplex

Simple Easy

HardComplex

Simple Easy

ComplexityObjective Subjective

"Elegance and familiarity are orthogonal." — Rich Hickey

Types of Complexity

● Essential Complexity

● Accidental Complexity

Examples of complexity

● State

● Inheritance

● Objects ○ state○ identity○ value

● Memory management● Variables

Why does simple matter?

● Formal reasoning○ can be hard

● Extendability○ Composing

● Testability○ isolation

How is Clojure simple(r)

● Functional Programming○ Pure functions

● Immutable data structures

● Thinking with data

● Decomplecting state / value / identity

Immutability

● Numbers are immutable in your language

● If you use Java, Strings are immutable

● Imagine if they're not!

Immutable building blocks

● Primitives: numbers, strings

● Collections: lists, vectors, strings, maps, sets

● "Changing" a collection creates an (efficient) copy. (think: Git)

● Don't worry about state

Thinking with data

● data is simple

● Why wrap it in classes

"It is better to have 100 functions operate on one data structure than 10 functions on 10 data structures." — Alan Perlis

Thinking with data

● data is simple

● Clojure map

Thinking with data

; Get the first name(get-in person [:name :first-name])

; Get all birth dates from a list of persons.(map :birthdate people)

Identity/state/value model

● Object Oriented Approach

SpaceShip

- name- x- y- velocity

ship1

Memory

● Object Oriented Approach

● Problems○ Stable value

○ history (snapshot)

○ Concurrent modification

Identity/state/value model

● Spaceship○ position

■ x,y■ Position object

○ velocity■ MovementState

Stable value

● Spaceship○ position

■ x,y■ Position object

○ velocity■ MovementState

○ ...

○ Consistent and stable object!

Stable value

● Spaceship○ position

■ x,y■ Position object

○ velocity■ MovementState

○ ...

○ Consistent and stable object! NO!

Stable value

Stable value

● Creating the object

● Not consistent

● Object Oriented Approach

● Problems○ Stable value

○ history (snapshot)

○ Concurrent modification

Identity/state/value model

Snapshot & history

● Take a picture

● Look later at it again.

● Object Oriented Approach

● Problems○ Stable value

○ history (snapshot)

○ Concurrent modification

Identity/state/value model

Concurrent modification

● Guarantee consistency?

● Stop the world!

Clojure approach

● Remove ambiguity of definitions○ value○ state○ identity

Identity/state/value model

● Value○ fixed: 42 , "answer"

● Identity○ series of different values over time

Me, a car,

● State○ Value of an identity at a certain point in time

Identity/state/value model

● OOP○ complects state and identity

● Clojure○ Separation value / state / identity

○ Perception becomes possible

○ Consistency management trivial

Clojure reference model

:name Heart Of Gold:position (0,0) :velocity 100 km/s

ship1

Value

Ref

Clojure reference model

:name Heart Of Gold:position (0,0) :velocity 100 km/s

ship1

Value

Ref

:name Heart Of Gold:position (100,100):velocity 100 km/s

Value

Clojure reference model

:name Heart Of Gold:position (0,0) :velocity 100 km/s

ship1

Value

Ref

:name Heart Of Gold:position (100,100) :velocity 100 km/s

Value

Clojure reference model

● Perception without stopping the world

● Atomic updates○ Transactions

Why Clojure?

Simplicity

Expressiveness

Concurrency

Why Clojure?Are we there yet?

http://www.infoq.com/presentations/Are-We-There-Yet-Rich-Hickey

Simple made easyhttp://www.infoq.com/presentations/Simple-Made-Easy

Out of The Tarpithttp://shaffner.us/cs/papers/tarpit.pdf

Why Clojure?

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