Clojure basics

CLOJURE BASICS
Kyle Oba
koba@pasdechocolat.com
@mudphone

Tuesday, March 19, 13

Clojure 1.5 was released on March 1st, 2013.

Read this to see changes:
https://github.com/clojure/clojure/blob/master/changes.md

My examples use 1.4.0.


“Startups tend to be an all or nothing
proposition. You either get rich, or you get
nothing. In a startup, if you bet on the
wrong technology, your competitors will
crush you.”
Paul Graham


“When you choose technology, you have to
ignore what other people are doing, and
consider only what will work the best.”

Paul Graham


“In a big company, you can do what all the
other big companies are doing. But a
startup can't do what all the other startups
do. I don't think a lot of people realize this,
even in startups.”

Paul Graham


“So if you're running a startup, you had
better be doing something odd. If not,
you're in trouble.”

Paul Graham


“Lisp is so great not because of some
magic quality visible only to devotees, but
because it is simply the most powerful
language available. And the reason
everyone doesn't use it is that
programming languages are not merely
technologies, but habits of mind as well,
and nothing changes slower.”

Paul Graham


“I'll begin with a shockingly controversial
statement: programming languages vary in
power.”

Paul Graham


http://pragprog.com/book/shcloj2/programming-clojure http://clojure.org


Rich Hickey is a genius. And, I’m just here to piss you off.

Clojure Concurrency
http://blip.tv/clojure/clojure-concurrency-819147
22:00 - 23:18

Simple Made Easy
http://www.infoq.com/presentations/Simple-Made-Easy
15:30 - 17:15


Why you should take a look...

functional langs can more easily use multiple cores

purely functional is awkward when dealing with state

Clojure has refs, agents, atoms, dynamic binding

dynamic typing

Java invocation (not functional)

refs (STM)

JVM, calling Java is idiomatic


CLOJURE BASICS

Why is Clojure so cool necessary?

Basic Setup x2 or 3

Language Basics

Working with State (or without it)

Working with government forms Java

Macros

Webby Stuﬀ

Testing

Clojure + Processing + Kinect/OpenNI


Why is Clojure necessary?


Multi-Core Future


Why use the JVM?

The JVM is advanced technology.

Java libraries... lots of them.

Widely deployed


Using the JVM
For example: https://github.com/mmcgrana/clj-redis

(require '[clj-redis.client :as redis])

(def db (redis/init))

(redis/ping db)
=> "PONG"

(redis/set db "foo" "BAR")
=> "OK"

(redis/get db "foo")
=> "BAR"


JVM

(ns clj-redis.client
(:import java.net.URI)
(:import (redis.clients.jedis Jedis JedisPool JedisPoolConfig JedisPubSub))
(:require [clojure.string :as str])
(:refer-clojure :exclude [get set keys type]))

(def ^{:private true} local-url
"redis://127.0.0.1:6379")

(defn init
([] (init {}))
([{:keys [url timeout test-on-borrow] :as opts}]
(let [uri (URI. (or url local-url))
tout (or timeout 2000)
host (.getHost uri)
port (if (pos? (.getPort uri)) (.getPort uri) 6379)
uinfo (.getUserInfo uri)
pass (and uinfo (last (str/split uinfo #":")))
config (JedisPoolConfig.)]
(when test-on-borrow
(.setTestOnBorrow config test-on-borrow))
(JedisPool. config host port tout pass)))
([k1 v1 & {:as opts}]
(init (assoc opts k1 v1))))


JVM

(defn lease [^JedisPool p f]
(let [j (.getResource p)]
(try
(f j)
(finally
(.returnResource p j)))))

(defn ping [p]
(lease p (fn [^Jedis j] (.ping j))))

(defn get [p ^String k]
(lease p (fn [^Jedis j] (.get j k))))

(defn set [p ^String k ^String v]
(lease p (fn [^Jedis j] (.set j k v))))


http://www.paulgraham.com/avg.html

Why is Clojure a secret weapon?


Why is Clojure so cool?

Homoiconic

Modern LISP

Functional (but not purely functional)


Homoiconic

Lisp code is just lisp data

Revenge of the Nerds - Paul Graham

Macros == black magic

Reprogramming the language with the language


Modern

Fewer parentheses

Sequences

Reader macros for data structures

regex, map, set, vector, metadata

First-class data structures: [], {}, ‘()

Less Lipsy than other Lisps (ex: fns use [ ])

Commas are whitespace


List Comprehension

user> (take 40
(for [x (range)
:when (and (> (* 2 x) 99)
(= 0 (mod x 3)))]
(* 2 x)))
(102 108 114 120 126 132 138 144 150 156
162 168 174 180 186 192 198 204 210 216
222 228 234 240 246 252 258 264 270 276
282 288 294 300 306 312 318 324 330 336)


Functional

Functions are ﬁrst-class

Data is immutable

Functions are NOT pure


Why Functional?

Simple

Thread-safe

Parallelizable

Generic


Clojure’s approach to Functional Programming

Not purely functional

Has system for working with refs, agents, atoms,
and dynamic binding

Dynamic typing

Java invocation is NOT functional


“Mutable objects are the new spaghetti code.”
- Rich Hickey


Concurrency is difﬁcult


You can’t stop the world

http://clojure.org/concurrent_programming

http://blip.tv/clojure/clojure-concurrency-819147

http://www.infoq.com/presentations/Value-Values


Concurrency is difﬁcult

Persistent data structures

Programming with values

Immutable by default

Syntax for state:

Refs (STM), Agents, Atoms, Dynamic Vars

We’ll a bit about state later.


Enough chit chat.


Let’s get started

there are a few options

Emacs, Leiningen, nrepl

Emacs Live

LightTable


Leiningen

Clojure automation...
https://github.com/technomancy/leiningen

1) Download the script:

https://raw.github.com/technomancy/leiningen/stable/bin/lein

2) Place it on your $PATH. (I like to use ~/bin)

3) Set it to be executable. (chmod 755 ~/bin/lein)

Start a new project:
$ lein new <project name>
$ lein repl


Emacs

Get version >= 24.X

Pick one (or more):

1) http://emacsformacosx.com

2) $ brew install emacs

3) https://github.com/overtone/emacs-live


Emacs https://gist.github.com/mudphone/4698169
= install lein 2
- Do this: https://github.com/technomancy/leiningen
- download lein script, it will boot strap itself
- install on path, rename to lein, chmod 755
- run lein --version
Leiningen 2.0.0 on Java 1.6.0_37 Java HotSpot(TM) 64-Bit Server VM

= Emacs >= 24
- Download latest version: http://emacsformacosx.com, if you want the GUI version
- Or, for CLI, use homebrew
- Or, do both

= Emacs Starter Kit
- look at my init.el
- install latest: https://github.com/technomancy/emacs-starter-kit
- install clojure-mode
package-install <ret> clojure-mode <ret>
- install nrepl
" package-install <ret> nrepl <ret>

= nrepl (already installed)
- docs here: https://github.com/kingtim/nrepl.el
- at a minimum, check out the keybindings

= nrepl auto-complete
- https://github.com/purcell/ac-nrepl
(popup docs keybinding didn't work for me, so I'm not using it right now)

= REFS:
- this isn't required reading, but it works:
http://www.kedrovsky.com/blog/clojure-emacs-nrepl-and-leiningen


Emacs

nREPL

M-x nrepl-jack-in

C-c M-n (to switch repl to this ns)

C-x C-c (eval buﬀer in repl)

M-C-x (eval form under point in repl)

C-c C-z (switch to repl buﬀer)


Clojure Basics


Getting Started

Symbols

Vars

Namespaces

use == require + refer

Help w/ doc, ﬁnd-doc, source


Forms

Preﬁx notation

false and nil evaluate to false

maps are functions of keys, and keys are functions of maps

defrecord


defrecord

user> (defrecord Person [fname lname age favorites])
user.Person
user> (defrecord Favorites [movie band])
user.Favorites
user> (def me (Person. "Kyle" "Oba" 37
(Favorites. "Lost in Translation" "Pantera")))
#'user/me
user> (:fname me)
"Kyle"

user> (-> me :favorites :movie)
"Lost in Translation"

user> (assoc me :fname "That guy")
#user.Person{:fname "That guy", :lname "Oba", :age 37,
:favorites #user.Favorites{:movie "Lost in Translation", :band "Pantera"}}

user> (update-in me [:favorites :band] #(str "ZZ" % "ZZ"))
#user.Person{:fname "Kyle", :lname "Oba", :age 37,
:favorites #user.Favorites{:movie "Lost in Translation", :band "ZZPanteraZZ"}}


http://clojure.org/state
State http://clojure.org/concurrent_programming

http://clojure.org/vars

http://clojure.org/atoms

http://clojure.org/agents

http://clojure.org/refs


Imperative programming

single-threaded premise

world is stopped

requires mutexes and locks

diﬃcult to get right / extremely complicated


Identity and Value

identity name fav foods today

{cheese
March18th
value Kyle donuts
2013
bacon}


Typical OO

has imperative programming baked into it

identities are conﬂated with values

not necessarily, but usually, due to the language


Clojure’s approach

separates state from identity

move away from state as “the content of this memory block”

toward “the value currently associated with this identity”

identity is in diﬀerent states at diﬀerent times

but, the state at a point in time never changes value


Clojure’s approach

vars - root binding, refers to mutable storage location

vars w/ dynamic binding - per-thread binding

atoms

agents, asynchronous change via a function and values

refs, coordinate change with STM: software transactional memory


Actor Model (not Clojure)

distributed

more complex

see Erlang


Working with State

atoms

refs

dynamic binding

acting at a distance / aspect oriented programming

loss of purity

memoize example


Working with State


vars allow reference to mutable storage locations

user> (def x 1)
#'user/x

user> x
1

user> (def y 1)
#'user/y

user> (+ x y)
2


dynamic binding

user> (def ^:dynamic x 1)
#'user/x

user> (def ^:dynamic y 1)
#'user/y

user> (+ x y)
2

user> (binding [x 2 y 3] (+ x y))
5

user> (+ x y)
2


Changing State async synchronous

coordinated ref

independent agent atom


atoms shared, synchronous, independent state

user> (def z (atom 5)) user> (swap! z inc)
#'user/z 6

user> z user> @z
#<Atom@f2882ad: 5> 6

user> @z user> (swap! z (constantly 10))
5 10

user> @z
10


agents independent, asynchronous change

user> (def a (agent 1)) user> (send a
#'user/a #(do (Thread/sleep 5000)
(+ % 1)))
user> a #<Agent@474d75ae: 2>
#<Agent@474d75ae: 1>
user> @a
2

user> @a
3
user> (send a inc)

user> a

user> @a
2


refs (transactional references)
shared use of mutable storage via STM

user> (def r (ref [1 2 3])) (dosync (alter r #(conj % 4)))
#'user/r [1 2 3 4]

user> r user> @r
#<Ref@369ca84f: [1 2 3]> [1 2 3 4]

user> @r
[1 2 3]

user> (dosync (alter r (partial cons 0)))
(0 1 2 3 4)

user> @r
(0 1 2 3 4)


Java Interop


Working with Java

Calling Java

Java interop

Java collections


Java: Examples

user> (System/getProperty "java.vm.version")
"20.14-b01-447"

user> (.toUpperCase "fred")
"FRED"

user> (.getName String)
"java.lang.String"

user> Math/PI
3.141592653589793


Java: import and .

user> (import '[java.util Random])
java.util.Random

user> (def r (Random.))
#'user/r

user> (.nextInt r)
458809484

user> (.nextInt r 100)
42


Java: doto

user> (doto (new java.util.HashMap) (.put "a" 1) (.put "b" 2))
#<HashMap {b=2, a=1}>


Java: set!

user> (def p (java.awt.Point.))
#'user/p

user> (.x p)
0

user> (set! (. p x) 5)
5

user> p
#<Point java.awt.Point[x=5,y=0]>


Macros


What’s a Macro?

#!/usr/bin/env ruby -w $ ruby ruby_macro.rb | sh
say_hello = "puts "hello"" hello
hello
ruby_cmd = (0..9).map do |n| hello
say_hello hello
end hello
hello
puts "ruby -e '#{ruby_cmd.join(";")}'" hello
hello
hello
hello

What if we could do this, while staying in the language?


Ruby-like “unless” implementation as a function...

(defn unless [expr form]
(if expr nil form))

user> (unless (= 1 1) (println "hi"))
hi
nil

This won’t work, because arguments are evaluated.


Arguments are evaluated ﬁrst

(defn unless [expr form]
(println "Doing unless...")
(if expr nil form))

user> (unless (= 1 1) (println "hi"))
hi
Doing unless...
nil


What we want

(unless expr form) -> (if expr nil form)


What we want

(defmacro unless [expr form]
(list 'if expr nil form))

user> (unless true (println "yep"))
nil

user> (unless false (println "yep"))
yep
nil


macroexpand-1

user> (macroexpand-1 '(when-not true (println "hi")))
(if true nil (do (println "hi")))

user> (source when-not)
(defmacro when-not
"Evaluates test. If logical false, evaluates body in an implicit do."
{:added "1.0"}
[test & body]
(list 'if test nil (cons 'do body)))
nil


WAT

text

data
reader evaluation macros
structures

compile

bytecode


macroexpand

user> (.. System (getProperties) (get "os.name"))
"Mac OS X"

user> (macroexpand-1 '(.. System (getProperties) (get "os.name")))
(.. (. System (getProperties)) (get "os.name"))

user> (macroexpand '(.. System (getProperties) (get "os.name")))
(. (. System (getProperties)) (get "os.name"))


Java Interop Bonus

user> (source ..)
(defmacro ..
"form => fieldName-symbol or (instanceMethodName-symbol args*)

Expands into a member access (.) of the first member on the first
argument, followed by the next member on the result, etc. For
instance:

(.. System (getProperties) (get "os.name"))

expands to:

(. (. System (getProperties)) (get "os.name"))

but is easier to write, read, and understand."
{:added "1.0"}
([x form] `(. ~x ~form))
([x form & more] `(.. (. ~x ~form) ~@more)))
nil


One more macro...

user> (source and)
(defmacro and
"Evaluates exprs one at a time, from left to right. If a form
returns logical false (nil or false), and returns that value and
doesn't evaluate any of the other expressions, otherwise it returns
the value of the last expr. (and) returns true."
{:added "1.0"}
([] true)
([x] x)
([x & next]
`(let [and# ~x]
(if and# (and ~@next) and#))))
nil


“Webby Stuff”


Areas of Interest:

ClojureScript to any old website
http://blip.tv/clojure/rich-hickey-unveils-clojurescript-5399498

ClojureScript + Three.js, to get at the WebGL bits

ClojureScript embedded in game systems

ClojureScript CLI

Pedestal: http://pedestal.io/documentation/hello-world-service/

It runs on Node.js.


ClojureScript
$ lein cljsbuild once
Compiling ClojureScript. $ tree
Compiling "resources/public/hello.js" from "src/cljs"... .
├── README.md
Successfully compiled "resources/public/hello.js" in 8.542318 seconds.
├── doc
│   └── intro.md
├── docroot
│   └── index.html
├── project.clj
├── resources
│   └── public
│   ├── hello.js
│   └── index.html
├── src
│   ├── clj
│   │   └── hello_world
│   │   └── core.clj
│   └── cljs
│   └── hello.cljs
└── test
└── hello_world
└── core_test.clj

10 directories, 9 ﬁles


Pedestal http://pedestal.io/documentation/hello-world-service/

$ tree
$ lein repl .
├── README.md
nREPL server started on port 63132 ├── conﬁg
... │   └── logback.xml
helloworld.server=> (use 'dev) ├── dev
nil │   ├── dev.clj
helloworld.server=> (start) │   └── user.clj
├── logs
nil
│   └── helloworld-2013-03-18.0.log
helloworld.server=> (stop)
├── project.clj
nil ├── src
helloworld.server=> Bye for now! │   └── helloworld
│   ├── server.clj
│   └── service.clj
├── target
│   ├── classes
│   └── stale
│   └── extract-native.dependencies
└── test
└── helloworld
└── service_test.clj

10 directories, 10 ﬁles


Pedestal http://pedestal.io/documentation/application-introduction/
(ns hello-world
(:require [io.pedestal.app.protocols :as p] (defn count-model [old-state message]
[io.pedestal.app :as app] (condp = (msg/type message)
[io.pedestal.app.messages :as msg] msg/init (:value message)
[io.pedestal.app.render :as render]
[domina :as dom]))
:inc (inc old-state)))

(defmulti render (fn [& args] (first args)))

(defmethod render :default [_] nil)

(defmethod render :value [_ _ old-value new-value]
(dom/destroy-children! (dom/by-id "content"))
(dom/append! (dom/by-id "content")
(str "<h1>" new-value " Hello Worlds</h1>")))

(defn render-fn [deltas input-queue]
(doseq [d deltas] (apply render d)))

(def count-app {:models {:count {:init 0 :fn count-model}}})

(defn receive-input [input-queue]
(p/put-message input-queue {msg/topic :count msg/type :inc})
(.setTimeout js/window #(receive-input input-queue) 3000))

(defn ^:export main []
(let [app (app/build count-app)]
(render/consume-app-model app render-fn)
(receive-input (:input app))
(app/begin app)))


Pedestal http://pedestal.io/documentation/application-introduction/
(ns hello-world (defn count-model [old-state message]
(:require [io.pedestal.app.protocols :as p]
[io.pedestal.app :as app] (condp = (msg/type message)
[io.pedestal.app.messages :as msg] msg/init (:value message)
[io.pedestal.app.render :as render] :inc (inc old-state)))
[io.pedestal.app.render.push :as push]
[domina :as dom]))
(defn render-value [renderer [_ _ old-value new-value] input-queue]
(dom/destroy-children! (dom/by-id "content"))
(dom/append! (dom/by-id "content")
(str "<h1>" new-value " Hello Worlds</h1>")))

(def count-app {:models {:count {:init 0 :fn count-model}}})

(defn receive-input [input-queue]
(p/put-message input-queue {msg/topic :count msg/type :inc})
(.setTimeout js/window #(receive-input input-queue) 3000))

(defn ^:export main []
(let [app (app/build count-app)
render-fn (push/renderer "content"
[[:value [:*] render-value]])]
(render/consume-app-model app render-fn)
(receive-input (:input app))
(app/begin app)))


Testing


Free!

(ns oudl.core-test user> (require '[clojure.test :as test])
(:use clojure.test nil
oudl.core))
user> (test/run-all-tests #"oudl.*-test")
(deftest a-test
(testing "FIXME, I fail." Testing oudl.core-test
(is (= 0 1))))
FAIL in (a-test) (core_test.clj:7)
FIXME, I fail.
expected: (= 0 1)
actual: (not (= 0 1))

Ran 1 tests containing 1 assertions.
1 failures, 0 errors.
{:type :summary, :pass 0, :test 1, :error 0, :fail 1}


(require ‘[clojure.test :as test])

user> (doc test/run-all-tests)
-------------------------
clojure.test/run-all-tests
([] [re])
Runs all tests in all namespaces; prints results.
Optional argument is a regular expression; only namespaces with
names matching the regular expression (with re-matches) will be
tested.
nil

user> (doc test/run-tests)
-------------------------
clojure.test/run-tests
([] [& namespaces])
Runs all tests in the given namespaces; prints results.
Defaults to current namespace if none given. Returns a map
summarizing test results.
nil


Things I Like


Some things

Clojure + Processing = Quil

Clojure + Kinect/OpenNI = Bifocals

LightTable


Clojure basics

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