Thinking Functionally with Clojure

Thinking Functionally with Clojure by John Stevenson @jr0cket www.practical.li

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

Complex Systems … are simply not easy to understand @jr0cket Josh Smith - Tweet Original Gif

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 Side Causes term coined by @krisajenkins

Pure Functions The 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 causes The 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

Clojure General 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”)

Clojure - basic syntax for this talk ( ) ;; an empty list. The first element of a list is evaluated as a function call (function-name data) ;; call a function with the data as its argument (def name “data-or-value”) ;; assign (bind) a name to a data or legal value :keyword-name ;; a keyword is a name that points to itself ;; Thread-first macro - chain function calls, passing the result of each call as the first argument to the next function. The ,,, indicates where the resulting argument goes. (-> (function-a “data”) (function-b ,,,) ;; In Clojure commas , are whitespace (function-c ,,, “data”))

Persistent data structures Built-in Immutability leading to deterministic code

List, Vector, Map & Set Clojure’s built-in data structures are all immutable - returning a new data structure when a function is applied (list 1 2 3 4 5) ‘(“fish” “chips” 42) (vec ‘(1 2 3 4)) [1 2 3 4] {:key “value”} {:name “John” :skill “conferencing”} (set ‘(1 2 3 4 4)) #{1 2 3 4}

Persistent Data Structures - shared memory Each function creates a new vector Memory space for values is shared between each vector

Persistent Data Structures - shared memory By sharing memory you can apply functions over and over again effectively Values persist until they are no longer referenced

Concurrency is Easier Concurrency is much easier to write and reason about because of - Pure functions - Immutability is encouraged by default - Persistent Data Structures - All values are immutable - unless explicitly wrapped in an atom or ref - state changes managed atomically (software transactional memory) - core.async library allows you to write asynchronous code as easily as sequential code

Using persistent data structures Iterate over collections Generate new collections by applying functions

Sequence / List Comprehension Iterate over collections

Sequence / List Comprehension Iterating through a range of generated values to create a list of 2 value vectors

Immutability - local binding Assignments made locally are immutable - words is a local binding to the result of running the function upper-case on “Hello World” - letter->clack is a function that converts a character to a code

Lazy Evaluation Only return a value when necessary ● maintain precision ● optimise evaluation

Lazy Evaluation Clojure's lazy sequences - returns a reference to a file and step through it one line at a time - line-seq returns a lazy sequence of lines, so we can read files larger than available memory, one line at a time

Polymorphism Functions evaluate different algorithms based on arity of arguments

Recursion & Polymorphism Process a collection of values by feeding the remaining elements back to the function - the sum function is polymorphic, it has different behaviours that could be evaluated depending on if passed 1 or 2 arguments

Recursion - tail call optimisation Protect the heap space from blowing by using the recur function Using recur in the last line is the same as calling sum, however the memory required from the previous sum function call is over-written in memory. So only 1 memory slot is used instead of 10 billion

Higher Order Functions A function that takes one or more functions as arguments, or a function that returns a function

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

Composing functions together Example: 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

Design idiom: Composing functions together You can think of functional design as evaluating one or more functions in series of functions

Functional Composition - map Map the function inc over each number of the collection, returns a new collection of numbers incremented by 1

Functional Composition - reduce Reduce the numbers inside a collection to a single value by applying the + function, returns a single value

Higher Order functions - partial, comp Higher order functions can return a function - The comp function composes other functions together - The partial function allows you to lazily add another argument to a function

Managing State State changes should be managed easily

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 updates The join-game-safely adds players to the ref and alters their account & game account

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

Parallelism Scale to the maximum

What is Parallelism In simple terms: run the same code across multiple CPU’s

Parallelism - pmap pmap - the same as map but in parallel

Parallelism - reduce Significantly lower completion time running in parallel using Immutable data structures https://github.com/aphyr/tesser

Parallelism - reducers & folds

Tools to learn Clojure inspire them & build up their motivation

Clojure support in many different tools

Leiningen - Clojure powered build automation

Figwheel (flappy birds example)

Examples, examples, examples we 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-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

Take your own journey into Clojure

Thank you @jr0cket jr0cket.co.uk

@jr0cket @Heroku @SalesforceDevs #Trailhead In a galaxy far, far away… London, UK

Thinking Functionally with Clojure

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