title: Symbolic Expression
role: manual
tags: #manual #reference #zettelstore
syntax: zmk
back: 00001012000000 00001012920516 00001012920525 00001012921000 00001012931000
backward: 00001012000000 00001012920516 00001012920525 00001012921000 00001012931000
box-number: 1
copyright: (c) 2020-present by Detlef Stern <ds@zettelstore.de>
created: 20230403145644
forward: 00001012930500
lang: en
license: EUPL-1.2-or-later
modified: 20230403154010
published: 20230403154010
visibility: public

A symbolic expression (also called __s-expression__) is a notation of a list-based tree.
Inner nodes are lists of arbitrary length, outer nodes are primitive values (also called __atoms__) or the empty list.

A symbolic expression is either
* a primitive value (__atom__), or
* a list of the form __(E,,1,, E,,2,, … E,,n,,)__, where __E,,i,,__ is itself a symbolic expression, separated by space characters.
An atom is a number, a string, or a symbol.

Symbolic expressions are used in programming languages like LISP or Scheme, where they denote both data structures and the program itself.
This allows a LISP or Scheme program to process LISP or Scheme programs.
That property is also used within Zettelstore.

Symbolic expressions are relatively easy to read, to parse, and to process.
=== See also
* [[Syntax|00001012930500]] of symbolic expressions in the Zettelstore
* [[S-expression @ Wikipedia|https://en.wikipedia.org/wiki/S-expression]]