Clojure Quick Reference

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

Also see the Clojure.org cheat-sheet

Calling functions

The first element in a list, (), is treated as a call to a function. This is known as prefix notation which greatly simplifies Clojure syntax and makes mathematical expressions completely deterministic, eliminating the need for operator precedence.

(+ 2 3 5 8 13 (* 3 7))
(+ 3 (* 2 (- 7 2) 4) (/ 16 4))
(clojure-version)

Functions contain doc-strings and you can ask for a functions documentation, or show the source code.

(doc doc)
(source doc)

Clojure is a dynamically typed language, it is also strongly typed (everything is a type, but you dont have to express the type in your code). The type of anything in Clojure can be returned.

(type 42)
(type {:hash "data" :map "more data"})

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"]

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

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

Defining names for values (vars)

Names can be bound to any values, simple values like numbers, collections or functions. A convenient way to refer to value in your code.

(def public-health-data
  ({:date "2020-01-01" :confirmed-cases 23014 :recovery-percent 15}
   {:date "2020-01-02" :confirmed-cases 23014 :recovery-percent 15}
   {:date "2020-01-03" :confirmed-cases 23014 :recovery-percent 15}))

(def add-hundred (partial + 100))

map reduce filter

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])

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.

Defining custom functions

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

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

(fn [x] (* x x))

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

Ratio Type

; Using the division function (/ ) shows another interesting characteristic of Clojure, the fact that it is lazy. This is not lazy in a bad way, but lazy evaluation of data structures. This actually helps to make clojure more efficient at dealing with data, especially very large data sets.

(/ 22 7)
22/7

(/ 22 7.0)
3.142857142857143

(type (/ 22 7))

Using a Ratio means that the mathematical division is not evaluated when using whole numbers (Integers) that would produce a decimal number. If you do return a decimal number then what precision of decimal are you expecting. By specifying one or more of the numbers as a decimal value you are giving Clojure a precision to infer and can therefore provide a specific decimal result.

Java interoperability

. and new are Clojure functions that create a Java object. This allows you to use values from Java constants, i.e. PI is a static double from the java.lang.Math object

(. Math PI)
3.141592653589793

Also call static and instance methods from Java objects.

(Math/cos 3)

(javax.swing.JOptionPane/showMessageDialog nil
    "Hello Java Developers")

Recursion

Recursive function

(defn recursive-counter
  [value]
  (if (< value 1000)
    (recur (+ value 25))))

(recursive-counter 100)