Special forms in Lisp

Abstract

Special forms are those expressions in the Lisp language which do not follow normal rules for evaluation. Some such forms are necessary as primitives of the language, while others may be desirable in order to improve readability, control the evaluation environment, implement abstraction and modularity, affect the flow of control, allow extended scoping mechanisms, define functions which accept a variable number of arguments, or achieve greater efficiency. There exist several long-standing mechanisms for specifying the definition of special forms: FEXPR's, NLAMBDA's and MACRO's.