path: root/clojure.html.markdown

---
language: clojure
filename: learnclojure.clj
contributors:
    - ["Adam Bard", "http://adambard.com/"]
---

Clojure is a Lisp family language developed for the Java Virtual Machine. It has
a much stronger emphasis on pure [functional programming](https://en.wikipedia.org/wiki/Functional_programming) than
Common Lisp, but includes several [STM](https://en.wikipedia.org/wiki/Software_transactional_memory) utilities to handle
state as it comes up.

This combination allows it to handle concurrent processing very simply,
and often automatically.

(You need a version of Clojure 1.2 or newer)


```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.

; The first call in a file should be ns, to set the namespace
(ns learnclojure)

; 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 stop it from
; being evaluated
'(+ 1 2) ; => (+ 1 2)
; (shorthand for (quote (+ 1 2)))

; You can eval a quoted list
(eval '(+ 1 2)) ; => 3

; Collections & Sequences
;;;;;;;;;;;;;;;;;;;

; Lists are linked-list data structures, while Vectors are array-backed.
; 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)

; "Collections" are just groups of data
; Both lists and vectors are collections:
(coll? '(1 2 3)) ; => true
(coll? [1 2 3]) ; => true

; "Sequences" (seqs) are abstract descriptions of lists of data.
; Only lists are seqs.
(seq? '(1 2 3)) ; => true
(seq? [1 2 3]) ; => false

; A seq need only provide an entry when it is accessed.
; So, seqs which can be lazy -- they can define infinite series:
(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)

; Conj will add an item to a collection in the most efficient way.
; For lists, they insert at the beginning. For vectors, they insert at the end.
(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 "Julie") ; => "Hello Julie"

; 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"


; Maps
;;;;;;;;;;

; Hash maps and array maps share an interface. Hash maps have faster lookups
; but don't retain key order.
(class {:a 1 :b 2 :c 3}) ; => clojure.lang.PersistentArrayMap
(class (hash-map :a 1 :b 2 :c 3)) ; => clojure.lang.PersistentHashMap

; Arraymaps will automatically become hashmaps through most operations
; if they get big enough, so you don't need to worry.

; Maps can use any hashable type as a key, but usually keywords are best
; Keywords are like strings with some efficiency bonuses
(class :a) ; => clojure.lang.Keyword

(def stringmap {"a" 1, "b" 2, "c" 3})
stringmap  ; => {"a" 1, "b" 2, "c" 3}

(def keymap {:a 1, :b 2, :c 3})
keymap ; => {:a 1, :c 3, :b 2}

; 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 key returns nil
(stringmap "d") ; => nil

; Use assoc to add new keys to hash-maps
(def newkeymap (assoc keymap :d 4))
newkeymap ; => {: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")


; Use the threading macros (-> and ->>) to express transformations of
; data more clearly.

; The "Thread-first" macro (->) inserts into each form the result of
; the previous, as the first argument (second item)
(->  
   {:a 1 :b 2} 
   (assoc :c 3) ;=> (assoc {:a 1 :b 2} :c 3)
   (dissoc :b)) ;=> (dissoc (assoc {:a 1 :b 2} :c 3) :b)

; This expression could be written as:
; (dissoc (assoc {:a 1 :b 2} :c 3) :b)
; and evaluates to {:a 1 :c 3}

; The double arrow does the same thing, but inserts the result of
; each line at the *end* of the form. This is useful for collection
; operations in particular:
(->>
   (range 10)
   (map inc)     ;=> (map inc (range 10)
   (filter odd?) ;=> (filter odd? (map inc (range 10))
   (into []))    ;=> (into [] (filter odd? (map inc (range 10)))
                 ; Result: [1 3 5 7 9]

; When you are in a situation where you want more freedom as where to
; put the result of previous data transformations in an 
; expression, you can use the as-> macro. With it, you can assign a
; specific name to transformations' output and use it as a
; placeholder in your chained expressions:

(as-> [1 2 3] input
  (map inc input);=> You can use last transform's output at the last position
  (nth input 2) ;=>  and at the second position, in the same expression
  (conj [4 5 6] input 8 9 10)) ;=> or in the middle !
                               ; Result: [4 5 6 4 8 9 10]


; 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
; Here, the module is clojure.string and the function is blank?
(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

; STM
;;;;;;;;;;;;;;;;;

; Software Transactional Memory is the mechanism clojure uses to handle
; persistent state. There are a few constructs in clojure that use this.

; An atom is the simplest. Pass it an initial value
(def my-atom (atom {}))

; Update an atom with swap!.
; swap! takes a function and calls it with the current value of the atom
; as the first argument, and any trailing arguments as the second
(swap! my-atom assoc :a 1) ; Sets my-atom to the result of (assoc {} :a 1)
(swap! my-atom assoc :b 2) ; Sets my-atom to the result of (assoc {:a 1} :b 2)

; Use '@' to dereference the atom and get the value
my-atom  ;=> Atom<#...> (Returns the Atom object)
@my-atom ; => {:a 1 :b 2}

; Here's a simple counter using an atom
(def counter (atom 0))
(defn inc-counter []
  (swap! counter inc))

(inc-counter)
(inc-counter)
(inc-counter)
(inc-counter)
(inc-counter)

@counter ; => 5

; Other STM constructs are refs and agents.
; Refs: http://clojure.org/refs
; Agents: http://clojure.org/agents
```

### Further Reading

This is far from exhaustive, but hopefully it's enough to get you on your feet.

Clojure.org has lots of articles:
[http://clojure.org/](http://clojure.org/)

Clojuredocs.org has documentation with examples for most core functions:
[http://clojuredocs.org/quickref/Clojure%20Core](http://clojuredocs.org/quickref/Clojure%20Core)

4Clojure is a great way to build your clojure/FP skills:
[http://www.4clojure.com/](http://www.4clojure.com/)

Clojure-doc.org (yes, really) has a number of getting started articles:
[http://clojure-doc.org/](http://clojure-doc.org/)

Clojure for the Brave and True has a great introduction to Clojure and a free online version:
[https://www.braveclojure.com/clojure-for-the-brave-and-true/](https://www.braveclojure.com/clojure-for-the-brave-and-true/)