S-expression

Summary

In computer programming, an S-expression (or symbolic expression, abbreviated as sexpr or sexp) is an expression in a like-named notation for nested list (tree-structured) data. S-expressions were invented for and popularized by the programming language Lisp, which uses them for source code as well as data. In the usual parenthesized syntax of Lisp, an S-expression is classically defined as

an atom of the form x, or

an expression of the form (x . y) where x and y are S-expressions.

This definition reflects LISP's representation of a list as a series of "cells", each one an ordered pair. In plain lists, y points to the next cell (if any), thus forming a list. The recursive clause of the definition means that both this representation and the S-expression notation can represent any binary tree. However, the representation can in principle allow circular references, in which cases the structure is not a tree at all, but a cyclic graph, and cannot be represented in classical S-expr

Official source

https://en.wikipedia.org/wiki/S-expression

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