Clojure intro talk outline

noco-webscale-clojure-intro-talk.md

A gentle introduction to Clojure

Ian Truslove

NoCo Web Scale

2015-12-01

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A gentle introduction to Clojure

"Lisp is over half a century old and it still has this perfect timeless air about it". Clojure is a Lisp. It's a dynamic programming language that targets the JVM and JavaScript.

It's also awesome.

In this talk I'll give you a really quick dive into Clojure programming, show you some of the most important language features and tools, and hopefully inspire you to dig a little deeper yourself.

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Intro

• My background
• It's gonna be quick in places - stop me at any time and ask for clarifications.
• Links (also at end):
  • Repo: github.com/iantruslove/gentle-intro
  • These slides: git.io/vB5Ov

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"Snippets" project: build a web service for sharing text snippets

(gist.github.com anyone?)

Interface:

$ curl -X POST \
    -H "content-type: text/plain" \
    -d "Hello, world!" \
    http://server/snippets/
# Should return HTTP 302 with a Location header for us to follow

$ curl -X GET http://server/snippets/1234
Hello, world!

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About Clojure

• It's (primarily) functional
• It's a Lisp
  • It's not John McCarthy's Lisp
• Dynamic
• Things you can do in Clojure / things inspired by Clojure
  • Distributed systems and web services
  • Apache Storm
  • Om - react.js but faster
  • Immutable.js
  • Overtone

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Functional Programming

• Think about verbs not nouns
  • See Steve Yegge's Execution in the Kingdom of Nouns
• Data "shape" is important
• Functions are first class

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Leiningen

• The de-facto build tool - http://leiningen.org/
  • but not the only build tool - Boot
• It's for creating and managing projects

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The REPL

• Read Evaluate Print Loop

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Editors

• A wealth of choices:
  • Emacs + clojure-mode + Cider
  • Vim + Fireplace + Cider
  • IntelliJ + Cursive
  • Nightcode
  • (?) Light Table
• Must-have features:
  • nREPL integration
  • Syntax highlighting
  • Auto paren matching
  • Structural navigation and editing

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The first hurdle

(Source: http://www.thejach.com/view/2011/8/short_guide_to_lisp_syntax)

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Hurdle? A curb, surely no more than that...

(Source: http://emacslife.com/how-to-read-emacs-lisp.html)

My advice: read from the inside out.

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Data and basic operations

• Numbers, strings, keywords
• def and immutability

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Collections and basic operations

• Lists, vectors, maps, sets
• seq ops
  • first, rest
  • conj
  • for, doseq
  • recursion
• map ops
  • assoc, dissoc
• map, filter
• reduce

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Immutable data

• Data don't change
• Structural sharing used for efficiency

(source: http://debasishg.blogspot.com/2010/05/grokking-functional-data-structures.html)

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Snippets: Intro to Ring

• Ring is a simple (conceptual) framework for building web applications.
  • It also has some useful code.
• Request maps:

{:server-port 8080
 :server-name "localhost"
 :uri "/foo/bar/baz"
 :scheme :https
 :headers {"content-type" "text/plain"
 :body <some Java stream object>}

• Response maps:

{:status 200
 :headers ["content-type" "text/plain"]
 :body "Hello, world!"}

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Functions

• fn
• defn
• Higher order functions
  • e.g. map takes a function as its second argument

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Common Code Structures

• Threading macros: -> and ->>
• let - local "variables"
• if, when, cond
• Destructuring

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Infinite computation

• Lazy seqs
• Recursion with (explicit) tail call optimization
  • Example: max of a list of numbers

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Organisation

• Namespaces, vars
• require to make other code available for use.

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Snippets: Ring Basics

• Creating and modifying responses
  • The functions in ring.util.response are useful

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Snippets: Ring Handlers

• Straightforward requirements:
  • Take a single parameter (request map)
  • Return a response map

(fn [req]
  {:status 200 :body ""})  

• Sketch out the GET and POST handlers...
  • ...in a Compojure defroutes setup

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Defining "things"

• plain maps
• deftype
• defrecord

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Polymorphism

• Multimethods
  • user-defined dispatch - typically on values
• Protocols
  • Type-based dispatch
  • Can extend (closed) Java classes to participate

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Unit testing

• clojure.test is the standard.
• some others:
  • Midje
  • Expectations

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Immutability By Default

• Design choices making state and concurrency easy to reason about pervade the language.
• Persistent data structures
  • "Persistent" as in it always preserves previous versions of itself, not as in a database.
• STM (Software Transactional Memory)

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Snippets: Storing snippets in an atom

• reset! and swap!

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Concurrency and Parallelism

• Lots of choices:
  • Threads
  • Futures
  • Promises
  • Agents
  • pmap (parallel map)
  • Reducers
  • JVM platform options, e.g. executors
  • core.async

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Interop

• Platform interop for Java and JavaScript

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Snippets: UUIDs

• We need them, Java has them.

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Snippets: Put it together

• Write the rest of the code!

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Snippets: Simple Extensions

• Web pages
• URL shortening
• Durable datastore
• Automatic content conversion (if possible)
  • e.g. JSON <--> EDN, JSON --> HTML, ...

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Resources

Websites

• Clojure for the Brave and True
  • now a book too
• Parens of the Dead

Books

• Living Clojure
• The Joy of Clojure

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More Resources

Other

• Slack - get access via clojurians.net
• IRC - on Freenode, #clojure, #clojure-beginners, #clojurescript, #clojure-emacs, ...
• Clojure Google Group
• Meetup groups:
  • Denver Den of Clojure
  • Boulder Clojurians

Conferences

• Clojure/conj
• Clojure/west
• Strange Loop

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End

• Links
  • Repo: github.com/iantruslove/gentle-intro
  • These slides: git.io/vB5Ov
  • Gist for slides: http://git.io/vB5Ya
• Me:
  • @iantruslove
  • https://github.com/iantruslove
  • [email protected]