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Clojure in 15 minutesλ︎

A quick tour of the Clojure syntax and common functions, which is so terse you can read through this page in around 15 minutes and have a basic understanding of the language.

Try the code out in the REPL

Start a Clojure REPL or use a Clojure aware editor connected to a REPL and experiment with these code examples.

Using the REPL provides instant feedback on each expression as they are evaluated, greatly increasing your understanding.


;; two semi-colons for a line comment, ; single semi-colon to comment the rest of the line

#_ comment reader macro to comment out the next form

(comment ) form to comment all the containing forms, useful to separate experimental and established code in a namespace.

Clojure expressionsλ︎

Clojure is mostly written with "expressions", a lists of elements inside parentheses, (), separated by space characters.

Clojure evaluates the first element in an expression as a function call. Additional elements in the expression are passed as value arguments to the called function.

Function call with value and expression as arguments

(+ 2007 (* 1 16))       

Functions can be passed as an argument

(map inc (range 0 99))  

Organising Clojureλ︎

Clojure code is organised into one or more namespaces. The namespace represents the directory path and file name that contains the code of the particular namespace.

A company name or community repository name is often used making the namespace unique and easier to share & reuse.

ns form returns nil value

The (ns namespace.,,,) expression returns a nil value, as its work is done behind the scenes.

All Clojure functions must return a value and nil is a value that means 'no value'.

Define a namespace


Define a longer namespace

(ns  ;; 
Namespaces use dash, directory and file names use underscore

Clojure uses kebab-case for names (common in Lisp dialects)

Unfortunately the Java Virtual Machine that hosts Clojure does not support dash, -, in file and directory names, so an underscore, -, character is used

String manipulationλ︎

The str function creates a new string from all the arguments passed

Combine strings into a single string value

(str "Hello" " " "World")

"Hello World" is returned from evaluating the expression.

clojure.string library for manipulating strings

clojure.string library functions manipulate values and return string values (other clojure.core functions my return characters as results, e.g. map)

Math, Truth & prefix notationλ︎

Functions use prefix notation, so you can do math with multiple values very easily

Prefix syntax takes multiple arguments

(+ 1 2 3 5 7 9 12) ; => 40

Math in Clojure is very precise, no need for operator precedence rules (as there are no operators)

Nesting forms defined a very precise calculation

Parentheses used instead of operator preceedence rules

(* 1 2 (- 24 (* 7 3)))

6 is returned as the value. Nested expressions are typically read inside out. (* 7 3) is 21, giving (- 24 21) expression resulting in 3. Finally the expression becomes (* 1 2 3), resulting in a value of 6

Maintain precision for calculations using a Ratio type in Clojure

Clojure Ratio value

(/ 27 7) 

22/7 is returned as the value, rather than a floating point value (double) which may loose some precision due to rounding.


= function provides a test for equality

Equal values return a boolean true

(= 1 1) ; => true

Unequals values return a boolean false

(= 2 1) ; => false

true and false are Boolean values and can be used literally in Clojure.


A predicate is a function that returns a boolean true or false value and by convention the function name ends in ?, e.g. true?, false?, seq?, even?, uuid?.

and & or functions can be used to chain the results of predicates together for more interesting conditional tests.

All predicates are true, returning true

(and (true? true) (not false)) ; => true

One of the predicates or values is true

(or nil (not= true false) (true? (complement true?)) ) ; => true

Truthy and Falsy values in Clojure

false boolean value and nil value are considered false in Clojure.

All other values are consider true.

Clojure Standard Library Predicate Functions

Collections & Sequencesλ︎

The most common data collections in Clojure:

  • (1 2 "three") or (list 1 2 "three") - a list of values read from start to end (sequential access)
  • [1 2 "three"] or (list 1 2 "three") - a vector of values with index (random access)
  • {:key "value"} or (hash-map :key "value") - a hash-map with zero or more key value pairs (associative relation)
  • #{1 2 "three"} or (set 1 2 "three") - a unique set of values

A list () is evaluated as a function call. The first element of the list the name of the function to call and additional values are arguments to the function.

The ' quote function informs the Clojure reader to treat the list as data only.

A quoted list is treated as data

'(1 2 3) ; => (1 2 3)

Lists and vectors are collections

(and (coll? '(1 2 3)) (coll? [1 2 3])) ; => true

Only lists are sequences

(seq? '(1 2 3)) ; => true
(seq? [1 2 3]) ; => false

Sequences are an interface for logical lists, which can be lazy. "Lazy" means that a sequence of valus are not evaluated until accessed.

A lazy sequence enables the use of large or even an infinite series, like so:

 (range) ; => (0 1 2 3 4 ...) - an infinite series
(take 4 (range)) ;  (0 1 2 3) - lazyily evaluate range and stop when enough values are taken

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 relative to the type of collection, 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))
; => [3 2 1]

Equivalent of (conj (conj (conj [] 3) 2) 1)

Annonymous Functionsλ︎

Use fn to create new functions that defines some behaviour. fn is referred to as an anonymous fuction as it has no external name to be referenced by and must be called within a list form.

(fn hello [] "Hello World") 

Wrap a (fn ,,,) form in parens to call it and return the result.

Call an anonymous function

((fn hello [] "Hello World")) ; => "Hello World"

Normally the anonymous function is used inline with other code

Use anonymous function within other code

(map (fn [x] (* x 2)) [1 2 3 4  [1 2 3 4 5]5])

Make the anonymous function reusable by binding it to a shared name (var) using def.

The var name bound to the function can now be called anywhere in the namespace.

As def creates a var (variable) name, the developer can changed the expression the name is bound to and re-evaluated to use the changed behaviour.

Bind a name to the anonymous function

(def hello-world 
  (fn hello [] "Hello World")) 

Evaluate annonymous function by evaluating its name


NOTE: hello-world is a name and not a function call, so parentheses are not required.

Shared Functionsλ︎

It is more common to use the defn macro to define a function. This is the same as defining the fn function and the def name within the same expression

Define a function with defn macro

(defn hello-world
  "I am a humble doc-string, please describe the function purpose"
 "Hello World")

#'user/hello-world is the value returned from evaluating the expression, showing the fully qualified name of the function. Note: the fully qualified name will be different when defined in a differnt namespace than user.

A defn function has the scope of the current namespace, so can be called anywhere in the namespace or in a namepace that has used require to include this namespace.

Call a function


The [] vector is used to define the argument names for the function. There can be zero or more arguments.

Call function with arguments

(defn hello [name]
  (str "Hello " name))

The correct number of arguments must be used when calling a function, or an error will be returned.

Call function with arguments

(hello "Steve") ; => "Hello Steve"
Pass a hash-map as an argument

Simplify the design of a function signature by passing all arguments as a hash-map.

(defn data-processing
  (let [body (get data :body)])
    (transform body))
🌐 Associative Destructuring can be used to automatically create local variables from the desired keys contained in the map, giving access to the value of each key.
(defn data-processing
  [{:keys [body]}]
  (transform body))

Clojure supports multi-variadic functions, allowing one function definition to respond to a function call with different number of arguments. This provides a simple form of polymorphism based on the number of arguments.

(defn hello-polly
  ([] "Hello World")  ; (1)!
  ([name] (str "Hello " name))) ; (2)!
  1. Call hello-polly with one argument

    (hello-polly "Jake") ; => "Hello Jake"

  2. Call hello-polly with zero arguments

    (hello-polly) ; => "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"

Hash-map collectionsλ︎

(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)
Commas are whitespace

commas are always treated as whitespace and are ignored by the Clojure reader

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. Strings cannot be used.

(:b keymap) ; => 2

("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}


(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 formsλ︎

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

  (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")

Namespaces and Librariesλ︎

Namespaces are used to organise code into logical groups. The top of each Clojure file has an ns form that defines the namespace name. The domain part of the namespace name is typically the organisation or community name (e.g. GitHub user/organisation)

(ns domain.namespace-name)

All Practicalli projects have namespace domains of practicalli

(ns practicalli.service-name)

require allows code from one namespace to be accessed from another namespace, either from a the same Clojure project or from a library added to the project classpath.

The :as directive with require is used to specify an alias name, a short-hand for the full library name

Or :refer [function-name var-name] can be used to specify specific functions and data (vars) that are available directly

A required directive is typically added to a namespace form

(ns practicalli.service-name
  (require [clojure.set :as set]))

The functions from clojure.set can be used via the alias name, rather than the fully qualified name, i.e. clojure.set/intersection

(set/intersection #{1 2 3} #{2 3 4}) ; => #{2 3}
(set/difference #{1 2 3} #{2 3 4}) ; => #{1}

:require directive can be used to include multiple library namespaces

(ns test
    [clojure.string :as string]
    [clojure.set :as set]))

require can be used by itself, usually within a rich code block

  (require 'clojure.set :as set))

Strong Dynamic Typesλ︎

Clojure is strongly typed, so everything is a type in Clojure.

Clojure is dynamically typed, so Clojure infers the type. A type does not need to be specified in the code, making the code simpler and more concise.

Clojure is a hosted language and uses the type system of the platform it runs upon. For example, Clojure uses Java object types for booleans, strings and numbers under the covers.

Use class or type function to inspect the type of some code in Clojure.

(type 1) ; Integer literals are java.lang.Long by default
(type 1.); Float literals are java.lang.Double
(type ""); Strings always double-quoted, and are java.lang.String
(type false) ; Booleans are java.lang.Boolean
(type nil); The "null" value is called nil

Vectors and Lists are java classes too!

(type [1 2 3]); => clojure.lang.PersistentVector
(type '(1 2 3)); => clojure.lang.PersistentList

Type hints

Type hints can be used to avoid reflection look-ups where performace critical issues have been identified. Type hints are not required in general. Clojure Type Hints

Java Interopλ︎

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 package

(import java.util.Date)

Or import from a java package name

(ns test

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

Last update: September 22, 2023