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.

Calling functions

The first element in a list, (), is treated as a call to a function. The examples show how to call functions with multiple arguments.

(+ 1 2)

(+ 3 (* 2 (- 7 2) 4) (/ 16 4))

(str "Clojure is " (- 2020 2007) " years old")

(inc 1)

(map inc [1 2 3 4 5])

(filter odd? (range 11))

Understanding functions

Functions contain doc-strings describing what that function does. The doc function returns the doc-string of a particular function. Most editors also support viewing of doc-strings as well as jumping to function definitions to view the 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.

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

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

{:key "value" "hash-map" "also referred to as dictionary"}

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

Defining names for values (vars)

Names can be bound to any values, from simple values like numbers, collections or even function calls. Using def is convenient way to create names for values that are shared in your code.

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}])

public-health-data

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 collecion 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? [])

Defining custom functions

(defn square-of
  "Calculates the square of a given number"
  [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 [x] (* x x)) [1 2 3 4 5])

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")

()

Next steps

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