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