Expressionexpressions

Expressionexpressions is a term used in discussions of programming languages and metaprogramming to describe expressions whose purpose is to produce or manipulate other expressions rather than directly evaluating to values. Although not part of formal language theory, the term appears in discussions of macro systems, staged computation, and code-as-data approaches.

Definition and scope: An expressionexpressions is an expression that, when evaluated, yields another expression or a fragment of code. This enables programs to generate, transform, or compose code fragments at compile time or run time rather than just computing a final result. The concept is often contrasted with ordinary expressions that produce concrete values immediately.

Purpose and usage: Expressionexpressions support metaprogramming by allowing languages to treat code as data and to construct new programs from smaller parts. They are central to macro systems, which manipulate code representations to produce expanded code, and to staged programming techniques, where expressions build code for later evaluation. In homoiconic languages, where code and data share the same representation, expressionexpressions are especially natural and powerful.

Examples and related ideas: In Lisp, macros operate on s-expressions, effectively building expressionexpressions that expand into new code. Template metaprogramming in languages like Haskell (via Template Haskell) and other staged languages provide mechanisms to generate code expressions for later compilation or execution.

See also: Macro (computer science); Metaprogramming; Code as data; Homoiconicity; Staged computation.