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Building Web Apps with Clojure
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Why Clojure?
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Based on Lisp
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"the greatest single programming language ever designed"
- Alan Kay, on Lisp
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"Lisp is worth learning for the profound enlightenment experience you will have
when you finally get it; that experience will make you a better programmer for the
rest of your days, even if you never actually use Lisp itself a lot."
- Eric Raymond, "How to Become a Hacker"
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"Lisp has jokingly been called "the most intelligent way to misuse a computer". I
think that description is a great compliment because it transmits the full flavor of
liberation: it has assisted a number of our most gifted fellow humans in thinking
previously impossible thoughts."
- Edsger Dijkstra
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• Originated in 1958. Second only to Fortran.
• The name LISP derives from LISt Processing.
• Lisp source code is made up of lists.
• Homoiconic: Code is data. Data is code.
(def lisp “Your grandpa’s language”)
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Infix Notation Prefix Notation
3 + 4 + 3 4
(5 - 6) * 7 * (- 5 6) 7
1 + 2 + 3 + 4 + 1 2 3 4
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C / Ruby / JS Lisp S-Expressions
3 + 4 (+ 3 4)
(5 - 6) * 7 (* (- 5 6) 7)
1 + 2 + 3 + 4 (+ 1 2 3 4)
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Lisp S-Expressions
=> (+)
0
=> (+ 2)
2
=> (+ 2 3)
5
=> (+ 2 3 4)
9
=> (+ 2 3 4 5)
14
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JavaScript Functions Clojure Functions
function sayHello(name) {
console.log("Hello, " + name + "!");
}
sayHello("Portland");
// Hello, Portland!
doSomethingWith(value1, value2);
(defn say-hello [name]
(println (str "Hello, " name "!")))
(say-hello "Portland")
; Hello, Portland!
(do-something-with value1 value2)
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REPL
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Functional
Functional
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First Class Functions
• pass functions as arguments
• higher order functions - return functions as values
• assign functions to variables
• store functions in data structures
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Pure Functions
• Same input always returns same output
• No side effects
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“A function is said to have a side effect if, in addition to returning a
value, it also modifies some state or has an observable interaction with
calling functions or the outside world.”
Side Effects
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• modify passed-in arguments
• change some internal or external state
• pass same arguments and yield different results
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Clojure is impure
Clojure is Impure
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Philosophy
“most parts of most programs should be functional, programs that are more
functional are more robust.”
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Immutable Data Structures
lists, vectors, maps, sets
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Primitives
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Primitives
• Numbers, Booleans, Nil, Strings
• Symbols - stand-in names for values, like constants
• Keywords - names not bound to a value
these
are
symbols
:these
:are
:keywords
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Lists
• Like arrays, but evaluated as a function (by default)
• First item is treated as a function name
• Remaining items are the arguments
• Can also be treated as data
=> (+ 1 2 3)
6
=> ‘(+ 1 2 3)
(+ 1 2 3)
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Vectors
• Zero-based arrays
• Can contain any value, and any mix of value types
• Not executed as code
=> [1 2 3]
[1 2 3]
=> [:a 0 “hello”]
[:a 0 “hello”]
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Maps
• Define a set of unique key-value pairs
• Comma separation optional for readability, treated the same
as whitespace
{:name “Clojure”
:functional true
:version “1.4.0”}
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Sets
• Collections of unique values
• Basic set operations like union / difference /intersection
#{:a :b :c :d}
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Iterators
each, map, filter, reduce, recur
map, filter, reduce, doseq, recur
Iterators
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Map
; increment each number
=> (map inc [1 2 3])
(2 3 4)
; anonymous function, add 10
=> (map #(+ 10 %) [1 2 3])
(11 12 13)
Applies a function to each element in a collection
and returns a new collection
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Reduce
=> (reduce + [2 3 4])
9
=> (reduce * [2 3 4])
24
Applies a function to all elements in a collection
and returns a value or collection
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Filter
=> (filter even? [1 2 3 4 5 6])
(2 4 6)
Applies a predicate function to each element in
a collection and returns a new filtered collection
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Hosted on the JVM
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Host Platforms: JVM, JS, .NET
• Clojure runs on the Java Virtual Machine.
• at near native Java speed
• with access to any library on the JVM
• Also runs on JavaScript, and .NET
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Built for Concurrency
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Simple Made Easy
http://www.infoq.com/presentations/Simple-Made-Easy-QCon-London-2012
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Simple Made Easy
• Emphasizing ease gives early speed
• Ignoring complexity will slow you down long term
• Many tools that are easy to use yield complex results
• Complect: to intertwine, entwine, or braid things
• Compose: to place together
• Composing simple components - the key to robust software
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State is never simple
• Complects value and time
• It is easy - in the sense that its at hand and familiar
• Interweaves everything that touches it
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Barriers
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Barriers
• Initial learning curve
• Prefix Notation, S-Expressions
• Functional vs Object Oriented
• Immutable data
• Recursion vs looping
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Build Something Real
• Something practical
• Learn by doing
• Crawl before you walk
• Canonical web examples: todo lists & blogs
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Editors
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Leiningen
The Clojure Project Automation tool
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Dependency Management
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Ring
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• Abstracts HTTP
• Similar to Ruby's Rack and Python's WSGI
• Handlers are functions that take requests at Clojure Maps
• and return responses as a Clojure Map
• Adapters run handlers on a web server
• Middleware augment handlers
Ring
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Request Map
{:uri
:query-string
:request-method
:headers
:body
... ... }
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Response Map
{:status
:headers
:body }
{:status 200
:headers {“Content-Type”: “text/html”}
:body “Hello Portland”}
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Example Hello World
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Lein-Noir
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Noir Project Structure
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Noir Views
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Generating HTML
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Hiccup
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Rendering Posts
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Data Persistence
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Simple SQL DSL
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It’s Just Data
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Testing
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Midje - Fact Checking
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Deployment
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Heroku
$ heroku create osbridge-clojure-blog
$ heroku addons:add shared-database
$ git push heroku master
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FIN
https://github.com/sbecker/osbridge-clojure-blog
Source Code
http://osbridge-clojure-blog.herokuapp.com
Demo Site
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Photo Credits
Lego Store - http://www.flickr.com/photos/vpickering/6297481250/
Clojure Logo - http://diego-pacheco.blogspot.com/2012/01/clojure-hell-yeah.html
Lisp on Paper - http://www.flickr.com/photos/phil-jackson/3044578328/
Functional Bag of Goodies - http://www.flickr.com/photos/beorn_ours/5675267679/
Side Effect Pedals - http://www.flickr.com/photos/terekhova/4629820574/
Dye Water Glasses - http://www.flickr.com/photos/pyth0ns/4816846174/
Galaxy - http://www.flickr.com/photos/skiwalker79/3855880846/
Series of Bullets - http://www.flickr.com/photos/dvids/7204296286/
Coffee Beans - http://www.flickr.com/photos/kubina/1469914113/sizes/l/in/photostream/
Bird Formation - http://www.flickr.com/photos/deapeajay/2694162918/
Great Wall - http://www.flickr.com/photos/franck-chilli/3882326984
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