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Fexpr
In Lisp programming languages, a fexpr is a function whose operands are passed to it without being evaluated. When a fexpr is called, only the body of the fexpr is evaluated; no other evaluations take place except when explicitly initiated by the fexpr. In contrast, when an ordinary Lisp function is called, the operands are evaluated automatically, and only the results of these evaluations are provided to the function; and when a (traditional) Lisp macro is called, the operands are passed in unevaluated, but whatever result the macro function returns is automatically evaluated.
==Origin of the name "fexpr"==
In early Lisp, the environment mapped each symbol to an association list, rather than directly to a value.〔McCarthy et al., ''Lisp I Programmer's Manual'', pp. 88–91.〕 Standard keys for these lists included two keys used to store a data value, to be looked up when the symbol occurred as an argument (APVAL and APVAL1); and four keys used to store a function, to be looked up when the symbol occurred as an operator. Of the function keys, SUBR indicated a compiled ordinary function, whose operands were evaluated and passed to it; FSUBR indicated a compiled special form, whose operands were passed unevaluated; EXPR indicated a user-defined ordinary function; and FEXPR indicated a user-defined special form. The only difference between a FEXPR and an EXPR was whether the operands were automatically evaluated.
In strict original usage, a FEXPR is therefore a user-defined function whose operands are passed unevaluated. However, in later usage the term ''fexpr'' may describe any first-class function whose operands are passed unevaluated, regardless of whether the function is primitive or user-defined.〔Pitman, ''The Revised MacLisp Manual'', p. 75.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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