Clojure Syntax

Clojure’s syntax, like of all Lisps, is very simple.


  • All Clojure code is written in a uniform structure
  • Clojure recognizes two kinds of structures:
    • Literal representations of data structures (like numbers, strings, maps and vectors)
    • Operations
  • Clojure evaluates every form to produce a value
  • All operations take the form opening parenthesis, operator, operands, closing parenthesis
    • ( operator operand1 operand2operandn )
    • Clojure uses whitespace to seperate operands

Control Flow


(if boolean-form 


  • The operator lets you wrap up multiple forms in parentheeses and run each of them
(if true
(do (println "Success!")
	"By Zeus's hammer!")
(do (println "Failure!")
	"By Aquaman's trident!"))


  • The when operator is like a combination of if and do, but with no else branch.
(when true
	(println "Success!")
	"abra cadabra")
  • Use when if you want to do multiple things when some condition is true, and you always want to return nil when the condition is false.

nil, true, flase, Truthiness, Equality and Boolean Expressions

  • nil is used to indicate no value.
    • You can check if a value is nil with the nil? function
 (nil? 1)
; => false
  • Equality operator in Clojure is =
(= 1 1)
; => true
  • Clojure uses boolean operators are or and and
    • or returns either the first truthy value or the last value
    • and returns the first falsey value value or, if no values are falsey, the last truthy value

Naming values with def

  • Use def to bind a name to a value
  • In other languages one would assign value to a variable and those languages usually encourage you to perform multiple assignments to the same variable
    • However in Clojure it is rarely needed to alter the name value association

Data Structures

  • Clojure comes with a handful of data structures you’ll use the majority of time
  • All Clojure’s data structures are immutable, meaning they cannot be changed at place


  • Clojure has quite sophisticated numerical support
  • Here’s an integer, a float and a ratio, respectively:


  • Strings represent text
  • Clojure only allows double quotes to delineate strings
"Lord Voldemort"
"\"He who must not be named\""
"\"Great cow of Moscow!\" - Hermes Conrad"
  • Clojure doesn’t have string interpolation
    • String concatenation is possible using the str function
(def name "Chewbacca")
(str "\"Uggllglglglglglglglll\" - " name)
; => "Uggllglglglglglglglll" - Chewbacca


  • Maps are a way of associating a value with some other value
    • They are similar to dictionaries or hashes in other languages
  • They are two kinds of maps in Clojure
    • Hash maps
    • Sorted maps
  • Maps can be of any type: strings, numbers, maps, vectors and even functions…!
  • Examples
    • empty map
      • {}
    • In this example, :first-name and :last-name are keywords
{:first-name "Charlie"
:last-name "McFishwich"}```
- Here we associate "string-key" with the `+` function:
{"string-key" +}
  • Maps can be nested
{:name {:first "John" :middle "Jacob" :last "Jingleheimerschmidt"}}

Working with maps

  • hash-map function can be used to create a map
(hash-map :a 1 :b 2)
; => {:a 1 :b 2}
  • You can look up values in a map with get function
(get {:a 0 :b 1} :b)
; => 1
(get {:a 0 :b {:c "ho hum"}} :b)
; => {:c "ho hum"}
  • The get-in function lets you look up values in nested maps
(get-in {:a 0 :b {:c "ho hum"}} [:b :c])
; => "ho hum"
  • Another way to look up a value in a map is to treat the map like a function with the key as its argument:
({:name "The Human Coffeepot"} :name)
; => "The Human Coffeepot"


  • They are primarily used as keys in maps
;; some keywords
;; you can look up :a in map
(:a {:a 1 :b 2 :c 3})
; => 1
  • Keywords are usually used as a function


  • A vector is similar to an array (it is a 0-indexed collection)
;; vector literal
[3 2 1]
;; Returning the 0th element of a vector
(get [3 2 1] 0)
; => 3
  • Vector elements can be of any type
  • Vectors can be created with the vector function
(vector "creepy" "full" "moon")
; => ["creepy" "full" "moon
  • You can use conj to add additional elements to the vector
(conj [1 2 3] 4)
; => [1 2 3 4]


  • Lists are similar to vectors in that they’re linear collection of values
    • '(1 2 3 4) - to write a list literal
  • If you want to retrieve elements from a list, use the nth function
(nth '(:a :b :c) 2)
; => :c
  • you cannot use get
    • get is more effective than nth so retrieving elements from a list is generally slower
  • List values can have any type
  • Lists can be created with the list function
(list 1 "two" {3 4})
; => (1 "two" {3 4})
  • Elements are added to the beginning of a list
(conj '(1 2 3) 4)
; => (4 1 2 3)


  • Sets are collection of unique values
  • Clojure has hash sets and sorted sets
  • Literal notation for hashed sets:
    • #{"kurt vonnegut" 20 :icicle}
  • Use hash-set to create a set
(hash-set 1 1 2 2)
; => #{1 2}
  • Multiple instances of a value become one unique value in a set
    • You can create sets from existing vectors and lists by using the set function
(set [3 3 3 4 4])
; => #{3 4}
  • Check for set membership using contains?
  • Use get or keywords for accessing members of a set


Calling Functions

  • As already mentioned, every Clojure operation has the same syntax: opening parenthesis, operator, operands, closing parentheses
  • Function call is just another term for an operation where the operator is a function or a function expression (an expression that returns a function)
    • e.g. function expression that return + (addition) function
(or + -)
; => #<core$_PLUS_ clojure.core$_PLUS_@76dace31>
((or + -) 1 2 3)
; => 6
  • Clojure’s support for first-class functions allows you to build very powerful abstractions
    • In other languages we think usually think of functions as generalizations of operations over data instances
      • e.g. the + function abstracts addition over any specific numbers.
    • By contrast, Clojure (and all Lisps) allows you to create functions that generalize over processes.
      • e.g. map allows you to generalize the process of transforming a collection by applying a function—any function—over any collection.
(map inc [0 1 2 3])
; => (1 2 3 4)
  • Clojure has no priviliged functions

Defining Functions

  • Functions are defined with the defn keyword
  • Functions are composed of five main parts:
      1. defn keyword
      1. Function name
      1. A docstring describing the function (optional)
      1. Parameters listed in brackets
      1. Function body
(defn too-enthusiastic
"Return a cheer that might be a bit too enthusiastic"


- Can be viewed in REPL with `doc` *fn-name*
- The docstring is also useful for generating documentation


  • Zero or more
    • The number of arguments is called arity
  • The values passed are called arguments
    • Arguments can be of any type
  • Functions support arity overloading
    • That is function can be defined so a a different function body will run with a different number of arguments
    • It is a way of providing default values tp arguments
(defn x-chop
"Describe the kind of chop you're inflicting on someone"
([name chop-type]
(str "I " chop-type " chop " name "! Take that!"))
(x-chop name "karate")))

Rest parameter

  • Functions support variable-arity by including a rest parameter
    • The rest parameter is indicated with an ampersand (&)
defn codger
[& whippersnappers]
(map codger-communication whippersnappers))

(codger "Billy" "Anne-Marie" "The Incredible Bulk")
; => ("Get off my lawn, Billy!!!"
;"Get off my lawn, Anne-Marie!!!"
;"Get off my lawn, The Incredible Bulk!!!")
  • You can mix rest parameters with normal parameters, but the rest has to come last


  • Lets you concisely bind names to values within a collection
;; Return the first element of a collection
(defn my-first
first-thing ; Notice that first-thing is within a vector

(my-first ["oven" "bike" "war-axe"])
; => "oven"

;; you can also destructure maps
(defn announce-treasure-location
[{lat :lat lng :lng}]
(println (str "Treasure lat: " lat))
(println (str "Treasure lng: " lng)))

Function body

  • Can contain forms of any kind
  • Clojure automatically returns the last form evaluated

Anonymous functions

  • Functions don’t need to have names
  • Anonymous functions are defined with the fn keyword
    • (fn [param-list] function body)
((fn [x] (* x 3)) 8)
; => 24```
- Anonymous function can be assigned with a name
	- e.g
		- ```clojure
(def my-special-multiplier (fn [x] (* x 3)))
(my-special-multiplier 12)
; => 36
  • Clojure has also a more compact way to create anonymous functions using reader macros (later on)
(#(str %1 " and " %2) "cornbread" "butter beans")
; => "cornbread and butter beans"
(#(identity %&) 1 "blarg" :yip)
; => (1 "blarg" :yip)

Returning functions

  • Functions can return other functions
  • The returned functions are closures
    • Which means that they can access all the variables that were in scope when the function was created
(defn inc-maker
"Create a custom incrementor"
#(+ % inc-by))

(def inc3 (inc-maker 3))

(inc3 7)
; => 10

Other tokens


  • Similar to using statement in C# for example
  • Helps you define a custom scope in which a name is bound to a value
  • You can reference existing bindings in let
    • You can use rest parameters in let as well
    • Two main use-cases
      • Clarity in naming things
      • Allows to evaluate a value only once and reuse the result
        • Important in the case of expensive function calls, e.g. to web APIs
(let [x 3]
; => 3

(def x 0)
(let [x (inc x)] x)
; => 1


  • Provides another way to do recursion in Clojure
  • loop [iteration 0] begins the loop and introduces a binding with an initial value
(loop [iteration 0]
(println (str "Iteration " iteration))
(if (> iteration 3)
(println "Goodbye!")
(recur (inc iteration))))
; => Iteration 0
; => Iteration 1
; => Iteration 2
; => Iteration 3
; => Iteration 4
; => Goodbye!
  • The same thing can be accomplished just by normal function definition
(defn recursive-printer
(recursive-printer 0))
(println iteration)
(if (> iteration 3)
(println "Goodbye!")
(recursive-printer (inc iteration)))))
; => Iteration 0
; => Iteration 1
; => Iteration 2
; => Iteration 3
; => Iteration 4
; => Goodbye!
  • But loop is less verbose and has better performance

Regular Expressions

  • The literal notation for a regular expression is to place the expression in quotes after a hash mark: #"regular-expression"
  • re-find function can be used to check whether text matches a pattern
(re-find #"^left-" "left-eye")
; => "left-"
(re-find #"^left-" "cleft-chin")
; => nil
(defn matching-part
{:name (clojure.string/replace (:name part) #"^left-" "right-")
:size (:size part)})
(matching-part {:name "left-eye" :size 1})
; => {:name "right-eye" :size 1}]
(matching-part {:name "head" :size 3})
; => {:name "head" :size 3}]