Munich Clojure Workshop - 01 - Introduction

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Introduction to Clojure Friday, October 11, 13

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Clojure, the language Friday, October 11, 13

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• Extremely expressive • First-class functions • Awesome Java Interoperability • Power of lisp (macros, function composition) • FUNctional Friday, October 11, 13

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What are we looking for? Friday, October 11, 13

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• sane development environment, integrated with a language • ability to develop and evaluate things on the fly • ease maintenance production • scalability • integration with well-known / existing tools • good, stable runtime • runtime debugging toolkit • performance Friday, October 11, 13

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• Leiningen, Clojure build tool https://github.com/technomancy/leiningen • repl just run `lein repl` in your console • emacs (it’s written in lisp, you know?) excellent nrepl and (older thing) clojure-swank, paredit-mode, formatting • nrepl get to the guts of runtime • VisualVm and the rest of Java goodness like jmap, jstack, jstat etc etc etc Friday, October 11, 13

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• JMX java management extensions • nrepl • JVM configuration/tuning • No shared mutable state • No locks • Pure functions -> same input -> same output • Easy composition and pipelining Friday, October 11, 13

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• Good baseline performance • Utilization of all processor resources anyone can eat out all the memory, right? • Painless parallel execution • Memory efficiency as far as it is possible with lisps, of course. in my mind that was mostly about structural sharing & persistent DS • JIT / inlining Friday, October 11, 13

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• Very, very good Java interop options • Most of libraries are either ported to Clojure, or use Java library underneath • Your favorite Java application container / web server • Jar packaging / public / private Maven repositories • Not to replace, but to complement your existing JVM toolkit Friday, October 11, 13

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Primer Friday, October 11, 13

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Functions Friday, October 11, 13

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function () { return 5; } JS Function Friday, October 11, 13

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(function () return 5) Add parentheses Friday, October 11, 13

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(fn [] return 5) function -> fn Friday, October 11, 13

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(fn [] 5) Ditch return Friday, October 11, 13

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Prefix notation, function calls Friday, October 11, 13

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(+ 1 1) Function Arguments Friday, October 11, 13

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Everything is an expression Friday, October 11, 13

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5 ; 㱺 5 Friday, October 11, 13

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(+ 1 1) ; 㱺 2 Friday, October 11, 13

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(if true "yes" "no") ; 㱺 “yes” Friday, October 11, 13

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Language Primitives Friday, October 11, 13

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"this is a string" String Friday, October 11, 13

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:key Keyword Friday, October 11, 13

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'key Symbol Friday, October 11, 13

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[1 2 3] [:key 1 "value"] Vector Friday, October 11, 13

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{:key "value" :key2 "value2"} Hash Friday, October 11, 13

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Syntactic Sugar Friday, October 11, 13

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#() => (fn []) Function Friday, October 11, 13

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#{} => (set ..) Set Friday, October 11, 13

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#inst "2013-10-04T13:32:41.008-00:00" Date Friday, October 11, 13

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Definitions Friday, October 11, 13

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(def a 1) Friday, October 11, 13

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(def a (+ 1 (* 2 2))) Friday, October 11, 13

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Functions Friday, October 11, 13

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(fn [number] (- number 1)) Function Definition Argument (s) Function Body Friday, October 11, 13

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(defn square "Returns a square of a number" [x] (* x x)) Function Definition Argument (s) Function Body Docstring Friday, October 11, 13

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Destructuring basics Friday, October 11, 13

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[1 2 3 ] [first second third] Friday, October 11, 13

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Passing functions as arguments to functions Friday, October 11, 13

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(map inc [1 2 3]) Function call Passed function Friday, October 11, 13

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(map (fn [i] (+ i 1)) [1 2 3]) Function call Passed (anonymous) function Friday, October 11, 13

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Getting help Friday, October 11, 13

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(clojure.repl/doc defn) Friday, October 11, 13

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(clojure.repl/source defn) Friday, October 11, 13

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Let Friday, October 11, 13

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function (a, b, c) { var sum = a + b + c; var count = 3; return sum / count; } Friday, October 11, 13

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(fn [a b c] (let [sum (+ a b c) count 3] (/ sum count)) Friday, October 11, 13

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Control structures Friday, October 11, 13

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Conditionals Friday, October 11, 13

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• Sometimes referred as “control structures” • Expressions, often implemented as macros • Used to delay evaluation of certain code blocks Friday, October 11, 13

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if Friday, October 11, 13

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Imagine “if” were a function for a minute: (if-fn (even? 2) (println "even!") (println "odd!")) Both “even!” and “odd!” are printed. Friday, October 11, 13

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`if` structure/usage: (if (predicate) (truthy-expression) (falsy-expression)) Friday, October 11, 13

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Multi-expression & do blocks (if (predicate) (do (truthy-expression) (another-truthy-expression)) (falsy-expression)) Friday, October 11, 13

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when Friday, October 11, 13

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(when (predicate) (expression-1) (expression-2)) Friday, October 11, 13

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• Useful when you don’t have “false” part • You can write more than one expression within `where` block Friday, October 11, 13

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if-let Friday, October 11, 13

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(def user {:name "Alex"}) (if-let [name (:name user)] (println "Hello " name) (println "Name is empty")) Friday, October 11, 13

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• Useful when you need to check an expression and reuse the expression result in “truth” block • As the name itself states, if-let behaves as let wrapped over if Friday, October 11, 13

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cond Friday, October 11, 13

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(cond (< n 0) "negative" (> n 0) "positive" :else "zero") Friday, October 11, 13

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• Useful when you want to avoid multiple nested ifs • `:else` here is arbitrary, you can use any expression that evaluates to something truthful Friday, October 11, 13

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Composition Friday, October 11, 13

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(fn [c] (first (rest c))) (comp first rest) Friday, October 11, 13

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Application Friday, October 11, 13

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(apply max [1 2 3]) (max 1 2 3) Friday, October 11, 13

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Namespaces Friday, October 11, 13

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(ns superlib.core) Friday, October 11, 13

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(:require ...) Friday, October 11, 13

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(:import ...) Friday, October 11, 13

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(:use ...) Friday, October 11, 13

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(:gen-class ...) Friday, October 11, 13

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Practical excercises Friday, October 11, 13

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• arrange into groups of 2-3 people • you’ll be given a function name and parameters vector • tests are located under `test/workshop_tasks/*-test.clj • write a function, run the tests • submit results to exercise server Friday, October 11, 13