Clojure: Magic Characters ' \ @ ^ # ` ~ .

By Xah Lee. Date: . Last updated: .

Here's list of clojure's special looking syntax.

Following are details.

x/y → slash used in identifier is a namespace separator. 〔►see Clojure: Namespace

*x* → by convention, names starting and ending with * usually means it's a builtin variable. Examples: clojure.core/*clojure-version* clojure.core/*command-line-args* clojure.core/*compile-files* clojure.core/*compile-path* clojure.core/*compiler-options* clojure.core/*err* clojure.core/*file* clojure.core/*flush-on-newline* clojure.core/*in* clojure.core/*ns* clojure.core/*out* clojure.core/*print-length*

*x → by convention, names starting with * but not ending with * usually means it's a builtin variable, whose value will change often. examples: clojure.core/*1 clojure.core/*2 clojure.core/*3 clojure.core/*e

x? → by convention, names ending with ? means it's a function that returns either true or false. e.g. clojure.core/empty?, clojure.core/even?

x! → by convention, names ending with ! means it's a function that modify a variable in-place. e.g. clojure.core/assoc!

x- → by convention, names ending with - means it creates a private function. e.g. clojure.core/defn- clojure.test/deftest-

:x → Clojure data type “clojure.lang.keyword”. Clojure “keyword” is 2 things:

::x → returns a keyword datatype but with the name x expanded to include namespace. This is usually used just like :key in a map data type, but when you want fully qualified names to avoid key clashes when 2 maps are merged from different libraries. See

(def x 3)
:x ; :x
::x ; :user/x

\chars → character escape. For example, {\a, \newline, \space, \tab, \formfeed, \backspace, \return}. Unicode characters are represented with \uNNNN as in Java. Octals are represented with \oNNN.

_ → No special meaning. _ is a allowed char for symbol. By convention, often used in function parameter as a name of a variable that is not used in function body. For example, (fn [x [_ y]] (+ x y)). This is often used when input (argument) is of a particular structure, thus the function parameters needs to match the shape, but only part of the value of the input is used in function body. 〔►see Clojure: Binding Forms, Destructuring

Function Parameter

[… & …] → for declaring optional parameters in function definition. For example, (defn my-length [& xx] (count xx)). clojure.core/&. 〔►see Clojure: Functions

% → used inside #(…) to stand for function parameters. % means first argument. %1 means first argument. %2 means second argument. For example, #(+ %1 %2) is equivalent to (fn [x y] (+ x y)) 〔►see Clojure: Functions

Function Chaining (Piping)

(-> …) → macro, for function chaining. Clojure calls it “threading”. clojure.core/->

(-> x f) is equivalent to (f x)

(-> x f g) is equivalent to (g (f x))

(->> …) → like unix pipe, but pass arg into the last argument position of function. clojure.core/->>

〔►see Clojure: Function Chaining

Dot Special Form, for Calling Java, outerClass$innerClass

(. …) → For calling Java's instance methods/field. (. object_name method_name args) → is Java's object_name.method_name(args) clojure.core/.

(.x a b …) → equivalent to (. a x b …) (Note: the dot appear before a identifier, with no space in between.)

(x. …) → equivalent to (new x …). For example, (java.util.Date.) is equivalent to (new java.util.Date) clojure.core/new (Note: the dot appear after a identifier, with no space in between.)

(.. o m1 m2) → linear syntax for method chaining of Java methods. For example, (.. System (getProperties) (get "")) is equivalent to (. (. System (getProperties)) (get "")) clojure.core/..

x$y → Access Java inner class. It means this EnclosingClass$NestedClass.

〔►see Clojure: Call Java Method


^{…} obj is same as (with-meta obj {…})

^:x obj is same as (with-meta obj {:x true})

^"…" obj is same as (with-meta obj {:tag "…"})

#^map form → add metadata map to form.

〔►see Clojure: Metadata


@x → equivalent to (deref x). This is known as “deference” clojure.core/deref

#char → “macro dispatch”. Whenever this char appears, it has special interpretation of adjacent syntax. The number sign char # causes Clojure “reader” to interpret adjacent syntax differently than normal. The character char following # is a “key” to a “reader table” that determines how it is to be interpreted.

#{…} → syntax for the datatype “set” (a collection of things). 〔►see Clojure: Collections (List, Vector, Set, Map, …), Sequence

#"regex" → Creates a regex object. The resulting object is of type java.util.regex.Pattern. 〔►see Clojure: regex

#(…) → equivalent to (fn […] (…)). Inside the parenthesis of #(…), % means first argument. %1 means first argument. %2 means second argument. For example, #(+ %1 %2) is equivalent to (fn [x y] (+ x y)). %& means rest arg. The #(…) form cannot be nested. 〔►see Clojure: Functions

#_… → Ignore next form. The form following #_ is completely skipped by the “reader”.

[4 #_5 6] ; [4 6]

[4 #_ 5 6] ; [4 6]

[4 #_ [7 8] 6] ; [4 6]

[4 #_ youdontknow 6] ; [4 6]

[4 #_ youdontknow# 6] ; [4 6]

[4 #_ (screwed not) 6] ; [4 6]

[4 #_ not) 6] ; runtime error

[4 #_ (not)) 6] ; runtime error

#x form → when # appear before a identifier thing such as variable name, it's known as “tagged literal”. Clojure will automatically call a function named x on form. There must be a file named named data_readers.clj at the root of the classpath. This file must contain a clojure map data type, that is, of the form {pair1 pair2 …}, where the key of the pair is a tag name, same as #x, and value is full name of a Clojure function. By default, clojure predefines some of the tagged literal. See

x# → when # appear after identifier, it causes Clojure to generate a new unique name internally, to avoid clash with other identifiers. This is usually used inside defmacro. clojure.core/gensym clojure.core/defmacro

Macros, Forms, Holding Evaluation

'form → equivalent to (quote form). It returns the un-evaluated form of form. clojure.core/quote

'(+ 1 2)
; returns (+ 1 2)

`(…) Syntax-quote. This will hold a expression in a un-evaluated form.

`(+ 1 2)
; returns (clojure.core/+ 1 2)

`(…) is similar to '(…), with the following exceptions:

• the symbols inside has their namespace attached. For example, `(list 3 4) becomes `(clojure.core/list 3 4)

• you can use ~ and ~@ to evaluate parts of the form inside a `(…)

(def x 3)

'(list x)
;; (list x)

`(list x)
;; (clojure.core/list xah-clojure-play.core/x)
(def x 3)

'(list ~x)
;; (list (clojure.core/unquote x))

`(list ~x)
;; (clojure.core/list 3)

~form→ syntax unquote. Used inside syntax quote `(…), will evaluate the form form

~@form→ unquote-splicing. Used inside syntax quote `(…), will evaluate the form form and remove its outer parenthesis.

~@ → Unquote-splicing. For all forms other than Symbols, Lists, Vectors, Sets and Maps, `x is the same as 'x.

For Symbols, syntax-quote resolves the symbol in the current context, yielding a fully-qualified symbol (i.e. namespace/name or fully.qualified.Classname). If a symbol is non-namespace-qualified and ends with #, it is resolved to a generated symbol with the same name to which _ and a unique id have been appended. For example, x# will resolve to x_123. All references to that symbol within a syntax-quoted expression resolve to the same generated symbol.

For Lists/Vectors/Sets/Maps, syntax-quote establishes a template of the corresponding data structure. Within the template, unqualified forms behave as if recursively syntax-quoted, but forms can be exempted from such recursive quoting by qualifying them with unquote or unquote-splicing, in which case they will be treated as expressions and be replaced in the template by their value, or sequence of values, respectively.

#'x → known as “var quote”. For example, #'x is equivalent to (var x). clojure.core/var

(def x 3) (var x) ; returns #'user/x

; → line comment.

, → optional separator, similar to space. Usually used in a map data type. For example: {:x 8, :y 9} is equivalent to {:x 8 :y 9} is also equivalent to {:x, 8, :y, 9}. [3,4] is equivalent to [3 4].