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.

In early Lisp, the environment mapped each symbol to an association list, rather than directly to a value. 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.

As a simple illustration of how fexprs work, here is a fexpr definition written in the Kernel programming language, which is similar to Scheme. (By convention in Kernel, the names of fexprs always start with $.)

This definition provides a fexpr called $f, which takes three operands. When the fexpr is called, a local environment is created by extending the static environment where the fexpr was defined. Local bindings are then created: symbols x, y, and z are bound to the three operands of the call to the fexpr, and symbol e is bound to the dynamic environment from which the fexpr is being called. The body of the fexpr, ($if ...), is then evaluated in this local environment, and the result of that evaluation becomes the result of the call to the fexpr. The net effect is that the first operand is evaluated in the dynamic environment, and, depending on whether the result of that evaluation is non-negative, either the second or the third operand is evaluated and that result returned. The other operand, either the third or the second, is not evaluated.

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