sexpressions

S-expressions, or symbolic expressions, are a notation for representing nested list data that is used to encode both program code and data in Lisp and many Lisp-inspired languages. An S-expression is either an atom or a list. A list is written as a sequence of one or more elements enclosed in parentheses, where each element can be an atom or another list. Atoms commonly include symbols, numbers, strings, and keywords.

Originating in the 1950s with John McCarthy for the Lisp language, S-expressions provide a simple, uniform syntax in which code and data share the same structure. This code-as-data property enables powerful macro systems and metaprogramming, as code can be generated and transformed using the same structural representation that represents data.

Syntax and evaluation: In Lisp, the meaning of an S-expression depends on context. A list beginning with a function or operator is evaluated by applying that function to the remaining elements; atoms are self-evaluating in many languages. Quoting can be used to create data instead of code, such as '(+ 2 3) to represent the list (+ 2 3) without evaluating it.

Usage and impact: S-expressions are central to Common Lisp, Scheme, Clojure, Emacs Lisp, and other Lisp dialects. They are valued for their minimal syntax and homoiconicity, the property that the primary data structure reflects the language's code structure. They have influenced data representation formats and tooling beyond Lisp.

Example: A simple function definition in Lisp style: (define (square x) (* x x)). A nested expression such as (+ 1 (* 2 3)) illustrates how lists encode operations and data.