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README.markdown
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README.markdown
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---
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noindex: true
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layout: home
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---
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# Learn X in Y minutes
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Whirlwind tours of (several, hopefully many someday) popular and
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ought-to-be-more-popular programming languages, presented as valid,
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commented code and explained as they go.
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## We need YOU!...
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... to write more inline code tutorials. Just grab an existing file from
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this repo and copy the formatting (don't worry, it's all very simple).
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Make a new file, send a pull request, and if it passes muster I'll get it up pronto.
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Remember to fill in the author and author\_url fields so you get credited
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properly!
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304
clojure.html.markdown
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---
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language: clojure
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author: Adam Bard
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---
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Clojure is rad
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```clojure
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; Comments start with semicolons.
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; Clojure is written in "forms", which are just
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; lists of things inside parentheses, separated by whitespace.
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;
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; The clojure reader assumes that the first thing is a
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; function or macro to call, and the rest are arguments.
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;
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; Here's a function that sets the current namespace:
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(ns test)
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; More basic examples:
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; str will create a string out of all its arguments
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(str "Hello" " " "World") ; => "Hello World"
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; Math is straightforward
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(+ 1 1) ; => 2
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(- 2 1) ; => 1
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(* 1 2) ; => 2
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(/ 2 1) ; => 2
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; Equality is =
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(= 1 1) ; => true
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(= 2 1) ; => false
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; You need not for logic, too
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(not true) ; => false
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; Nesting forms works as you expect
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(+ 1 (- 3 2)) ; = 1 + (3 - 2) => 2
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; Types
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;;;;;;;;;;;;;
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; Clojure uses Java's object types for booleans, strings and numbers.
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; Use `class` to inspect them.
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(class 1) ; Integer literals are java.lang.Long by default
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(class 1.); Float literals are java.lang.Double
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(class ""); Strings always double-quoted, and are java.lang.String
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(class false) ; Booleans are java.lang.Boolean
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(class nil); The "null" value is called nil
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; If you want to create a literal list of data, use ' to make a "symbol"
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'(+ 1 2) ; => (+ 1 2)
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; You can eval symbols.
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(eval '(+ 1 2)) ; => 3
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; Collections & Sequences
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;;;;;;;;;;;;;;;;;;;
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; Vectors and Lists are java classes too!
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(class [1 2 3]); => clojure.lang.PersistentVector
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(class '(1 2 3)); => clojure.lang.PersistentList
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; A list would be written as just (1 2 3), but we have to quote
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; it to stop the reader thinking it's a function.
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; Also, (list 1 2 3) is the same as '(1 2 3)
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; Both lists and vectors are collections:
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(coll? '(1 2 3)) ; => true
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(coll? [1 2 3]) ; => true
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; Only lists are seqs.
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(seq? '(1 2 3)) ; => true
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(seq? [1 2 3]) ; => false
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; Seqs are an interface for logical lists, which can be lazy.
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; "Lazy" means that a seq can define an infinite series, like so:
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(range 4) ; => (0 1 2 3)
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(range) ; => (0 1 2 3 4 ...) (an infinite series)
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(take 4 (range)) ; (0 1 2 3)
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; Use cons to add an item to the beginning of a list or vector
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(cons 4 [1 2 3]) ; => (4 1 2 3)
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(cons 4 '(1 2 3)) ; => (4 1 2 3)
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; Use conj to add an item to the beginning of a list,
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; or the end of a vector
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(conj [1 2 3] 4) ; => [1 2 3 4]
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(conj '(1 2 3) 4) ; => (4 1 2 3)
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; Use concat to add lists or vectors together
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(concat [1 2] '(3 4)) ; => (1 2 3 4)
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; Use filter, map to interact with collections
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(map inc [1 2 3]) ; => (2 3 4)
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(filter even? [1 2 3]) ; => (2)
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; Use reduce to reduce them
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(reduce + [1 2 3 4])
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; = (+ (+ (+ 1 2) 3) 4)
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; => 10
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; Reduce can take an initial-value argument too
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(reduce conj [] '(3 2 1))
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; = (conj (conj (conj [] 3) 2) 1)
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; => [3 2 1]
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; Functions
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;;;;;;;;;;;;;;;;;;;;;
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; Use fn to create new functions. A function always returns
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; its last statement.
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(fn [] "Hello World") ; => fn
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; (You need extra parens to call it)
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((fn [] "Hello World")) ; => "Hello World"
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; You can create a var using def
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(def x 1)
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x ; => 1
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; Assign a function to a var
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(def hello-world (fn [] "Hello World"))
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(hello-world) ; => "Hello World"
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; You can shorten this process by using defn
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(defn hello-world [] "Hello World")
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; The [] is the list of arguments for the function.
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(defn hello [name]
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(str "Hello " name))
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(hello "Steve") ; => "Hello Steve"
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; You can also use this shorthand to create functions:
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(def hello2 #(str "Hello " %1))
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(hello2 "Fanny") ; => "Hello Fanny"
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; You can have multi-variadic functions, too
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(defn hello3
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([] "Hello World")
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([name] (str "Hello " name)))
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(hello3 "Jake") ; => "Hello Jake"
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(hello3) ; => "Hello World"
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; Functions can pack extra arguments up in a seq for you
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(defn count-args [& args]
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(str "You passed " (count args) " args: " args))
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(count-args 1 2 3) ; => "You passed 3 args: (1 2 3)"
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; You can mix regular and packed arguments
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(defn hello-count [name & args]
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(str "Hello " name ", you passed " (count args) " extra args"))
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(hello-count "Finn" 1 2 3)
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; => "Hello Finn, you passed 3 extra args"
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; Hashmaps
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;;;;;;;;;;
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(class {:a 1 :b 2 :c 3}) ; => clojure.lang.PersistentArrayMap
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; Keywords are like strings with some efficiency bonuses
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(class :a) ; => clojure.lang.Keyword
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; Maps can use any type as a key, but usually keywords are best
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(def stringmap (hash-map "a" 1, "b" 2, "c" 3))
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stringmap ; => {"a" 1, "b" 2, "c" 3}
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(def keymap (hash-map :a 1 :b 2 :c 3))
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keymap ; => {:a 1, :c 3, :b 2} (order is not guaranteed)
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; By the way, commas are always treated as whitespace and do nothing.
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; Retrieve a value from a map by calling it as a function
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(stringmap "a") ; => 1
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(keymap :a) ; => 1
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; Keywords can be used to retrieve their value from a map, too!
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(:b keymap) ; => 2
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; Don't try this with strings.
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;("a" stringmap)
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; => Exception: java.lang.String cannot be cast to clojure.lang.IFn
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; Retrieving a non-present value returns nil
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(stringmap "d") ; => nil
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; Use assoc to add new keys to hash-maps
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(assoc keymap :d 4) ; => {:a 1, :b 2, :c 3, :d 4}
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; But remember, clojure types are immutable!
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keymap ; => {:a 1, :b 2, :c 3}
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; Use dissoc to remove keys
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(dissoc keymap :a :b) ; => {:c 3}
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; Sets
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;;;;;;
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(class #{1 2 3}) ; => clojure.lang.PersistentHashSet
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(set [1 2 3 1 2 3 3 2 1 3 2 1]) ; => #{1 2 3}
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; Add a member with conj
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(conj #{1 2 3} 4) ; => #{1 2 3 4}
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; Remove one with disj
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(disj #{1 2 3} 1) ; => #{2 3}
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; Test for existence by using the set as a function:
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(#{1 2 3} 1) ; => 1
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(#{1 2 3} 4) ; => nil
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; There are more functions in the clojure.sets namespace.
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; Useful forms
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;;;;;;;;;;;;;;;;;
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; Logic constructs in clojure are just macros, and look like
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; everything else
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(if false "a" "b") ; => "b"
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(if false "a") ; => nil
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; Use let to create temporary bindings
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(let [a 1 b 2]
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(> a b)) ; => false
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; Group statements together with do
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(do
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(print "Hello")
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"World") ; => "World" (prints "Hello")
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; Functions have an implicit do
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(defn print-and-say-hello [name]
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(print "Saying hello to " name)
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(str "Hello " name))
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(print-and-say-hello "Jeff") ;=> "Hello Jeff" (prints "Saying hello to Jeff")
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; So does let
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(let [name "Urkel"]
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(print "Saying hello to " name)
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(str "Hello " name)) ; => "Hello Urkel" (prints "Saying hello to Urkel")
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; Modules
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;;;;;;;;;;;;;;;
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; Use "use" to get all functions from the module
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(use 'clojure.set)
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; Now we can use set operations
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(intersection #{1 2 3} #{2 3 4}) ; => #{2 3}
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(difference #{1 2 3} #{2 3 4}) ; => #{1}
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; You can choose a subset of functions to import, too
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(use '[clojure.set :only [intersection]])
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; Use require to import a module
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(require 'clojure.string)
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; Use / to call functions from a module
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(clojure.string/blank? "") ; => true
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; You can give a module a shorter name on import
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(require '[clojure.string :as str])
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(str/replace "This is a test." #"[a-o]" str/upper-case) ; => "THIs Is A tEst."
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; (#"" denotes a regular expression literal)
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; You can use require (and use, but don't) from a namespace using :require.
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; You don't need to quote your modules if you do it this way.
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(ns test
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(:require
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[clojure.string :as str]
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[clojure.set :as set]))
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; Java
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;;;;;;;;;;;;;;;;;
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; Java has a huge and useful standard library, so
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; you'll want to learn how to get at it.
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; Use import to load a java module
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(import java.util.Date)
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; You can import from an ns too.
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(ns test
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(:import java.util.Date
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java.util.Calendar))
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; Use the class name with a "." at the end to make a new instance
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(Date.) ; <a date object>
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; Use . to call methods. Or, use the ".method" shortcut
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(. (Date.) getTime) ; <a timestamp>
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(.getTime (Date.)) ; exactly the same thing.
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; Use / to call static methods
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(System/currentTimeMillis) ; <a timestamp> (system is always present)
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; Use doto to make dealing with (mutable) classes more tolerable
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(import java.util.Calendar)
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(doto (Calendar/getInstance)
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(.set 2000 1 1 0 0 0)
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.getTime) ; => A Date. set to 2000-01-01 00:00:00
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```
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