Clojure Quick Reference

The basic Clojure syntax and a few common functions you should probably learn first.

The examples are editable (using an embedded REPL) so feel free to experiment and watch as the return value changes as you change the code. Reload the page if you want to reset all the code back to the starting point.

Install Clojure on your computer if you want to experiment even further.

Want to go deeper already?

Watch the Clojure language video series by Brian Will for a detailed introduction to key parts of the language. Or discover Clojure core functions by completing challenges on and then watching how Practicalli solved them.

Calling functions

The first element in a list, (), is a call to a function. Any other elements are passed to the function as arguments. The examples show how to call functions with multiple arguments.

(+ 1 2)
(+ 3 (* 2 (- 7 2) 4) (/ 16 4))
(str "Clojure is " (- 2021 2007) " years old")
(inc 1)
(map inc [1 2 3 4 5])
(filter odd? (range 11))

Prefix notation and parens

Hugging code with () is a simple syntax to define the scope of code expressions. No additional ;, , or spaces are required.

Treating the first element of a list as a function call is referred to as prefix notation, which greatly simplifies Clojure syntax. Prefix notation makes mathematical expressions completely deterministic, eliminating the need for operator precedence.

Understanding functions

The clojure.repl/doc function returns the doc-string of the given function. A doc-string should be part of all public function definitions. Clojure editors should provide commands to view doc-strings and the ability to jump to function definitions to view their source code

(doc doc)

Modeling data with Collection types

Clojure has 4 main collection types, all immutable (cannot change once created) and can contain any Clojure types.

A list, (), used for calling functions and representing sequences. A linked list for sequential access.

(str "lists used mainly " (* 2 2) " " :code)

A vector, [], used for simple collections of values. An indexed data structure for random access

[0 "indexed" :array (* 2 2) "random-access" 4 :data]

A map, {}, use for descriptive data collections. An associative data structure for value lookup by unique keys (also known as a dictionary).

{ :hash-map :associative-collection :pairs {:key "value"} :aka "dictionary"}

A set, #{}, use as a unique set of values. Sets are used to test if a value is contained within, i.e. predicates.

#{1 2 3 4 "unique" "set" "of" "values" "unordered" (* 3 9)}

Persistent data types

Values are immutable so when a function changes a value a new immutable value is created. When creating new collection values, unchanged values are shared with the original collection. This sharing model is called persistent data types and enables immutable data to be used efficiently.

Using data structures

Using the map and inc function, increment all the numbers in a vector

(map inc [1 2 3 4 5])

The above map function is roughly equivalent to the following expression

(conj [] (inc 1) (inc 2) (inc 3) (inc 4) (inc 5))

The conj function creates a new collection by combining a collection and one or more values.

map reduce filter are common functions for iterating through a collection / sequence of values

(map * [1 3 5 8 13 21] [3 5 8 13 21 34])
(filter even? [1 3 5 8 13 21 34])
(reduce + [31 28 30 31 30 31])
(empty? [])

Many Clojure core functions for collections

map, reduce, apply, filter, remove are just a few examples of Clojure core functions that work with data structures.

Defining custom functions

(defn square-of
  "Calculates the square of a given number"
  (* number number))

(square-of 9)

Function definitions can also be used within other expressions, useful for mapping custom functions over a collection

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

Defining local names

Use the let function as a simple way to experiment with code designs

(let [data (range 24 188)
      total (reduce + data)
      values (count data)]
  (str "Average value: " (/ total values)))

Define local names to remove duplication in function definitions, or to simplify algorithms

(defn square-of
  "Calculates the square of a given number"
  (* number number))

(square-of 9)

Defining names for values (vars)

A name bound to a value can be used to represent that value throughout the code. Names can be bound to simple values (numbers, strings, etc.), collections or even function calls.

def binds a name to a value with the scope of the current namespace. def is useful for data that is passed to multiple functions within a namespace.

Evaluating a name will return the value it is bound to.

(def public-health-data
  [{:date "2020-01-01" :confirmed-cases 23814 :recovery-percent 15}
   {:date "2020-01-02" :confirmed-cases 24329 :recovery-percent 14}
   {:date "2020-01-03" :confirmed-cases 25057 :recovery-percent 12}])


def for shared values, let for locally scoped values

let function is used to bind names to values locally, such as within a function definition. Names bound with def have namespace scope so can be used with any code in that namespace.

Iterating over collections

map iterates a function over a collection of values, returning a new collection of values

(map inc (range 20))

reduce iterates a function over the values of a collection to produce a new result

(reduce + (range 101))

Reducing functions are function definitions used by the reduce function over a collection

(reduce (fn [[numerator denominator] accumulator]
          [(+ numerator accumulator)
           (inc denominator)])
        [0 0]
        (range 1 20))

Functions can call themselves to iterate over a collection. Using a lazy sequence means only the required numbers are generated, ensuring efficiency of operation and making the function usable in many different scenarios.

(defn fibonacci-sequence
  [current-number next-number]
    (cons current-number
          (fibonacci-sequence next-number (+ current-number next-number)))))

(take 10 (fibonacci-sequence 0 1))

Host Interoperability

The REPL in this web page is running inside a JavaScript engine, so JavaScript functions can be used from within ClojureScript code (ClojureScript is Clojure that runs in JavaScript environments).

In the box below, replace () with (js/alert "I am a pop-up alert")


Java libraries in Clojure

java.lang library is available in Clojure by default and many other Java methods can be included by using their full name, e.g. (java.lang.Date.) will return the current date.

Next steps

Install Clojure on your computer if you want to experiment even further or keep on reading more about Clojure.

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