lexpressions

Lexpressions are a notation used in Lisp-like programming languages to represent code and data. Similar to S-expressions, an Lexpression is typically written as a nested, parenthesized list, where the first element denotes a function, operator, or macro, and the remaining elements are arguments. The prefix form makes Lexpressions both source code and data structures, a characteristic known as homoiconicity. In many overviews the terms Lexpressions and S-expressions are used interchangeably, though some authors reserve Lexpressions for language-specific constructs.

Structure and evaluation: An Lexpression's meaning depends on the language. In a typical Lisp-like system, evaluating a list calls the function named by the first element with the evaluated rest as arguments, unless the form is a special form or macro. Quotes prevent evaluation; macros transform Lexpressions at compile or macro-expansion time.

Examples: A simple function application is (+ 2 3). A function definition might be (defun square (x) ( x x)). A lambda example is ((lambda (x) ( x x)) 5). A quoted literal is '(+ 1 2). A conditional example is (if (> x 0) x (- x)).

Use cases and history: Lexpressions originated with the design of Lisp in the late 1950s and underpin metaprogramming, macros, domain-specific languages, and interactive development in REPL environments. They are used across many dialects, including Common Lisp, Scheme, and Clojure, and remain a core concept in discussions of code-as-data and macro systems.

See also: Lisp, S-expression, homoiconicity, macro.