---
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.) ;
; Use . to call methods. Or, use the ".method" shortcut
(. (Date.) getTime) ;
(.getTime (Date.)) ; exactly the same thing.
; Use / to call static methods
(System/currentTimeMillis) ; (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
```