Macro define-symbol-macro

Syntax:

Arguments and Values:

symbol—a symbol.

expansion—a form.

Description:

Provides a mechanism for globally affecting the macro expansion of the indicated symbol.

Globally establishes an expansion function for the symbol macro named by symbol. Patterned after wording from SYMBOL-MACROLET (per issue SYMBOL-MACROLET-SEMANTICS).The only guaranteed property of an expansion function for a symbol macro is that when it is applied to the form and the environment it returns the correct expansion. (In particular, it is implementation-dependent whether the expansion is conceptually stored in the expansion function, the environment, or both.)

Each global reference to symbol (i.e., not shadowed₂ by a binding for a variable or symbol macro named by the same symbol) is expanded by the normal macro expansion process; see Section 3.1.2.1.1 (Symbols as Forms). The expansion of a symbol macro is subject to further macro expansion in the same lexical environment as the symbol macro reference, exactly analogous to normal macros.

Patterned after wording from SYMBOL-MACROLET (per issue SYMBOL-MACROLET-DECLARE).The consequences are unspecified if a special declaration is made for symbol while in the scope of this definition (i.e., when it is not shadowed₂ by a binding for a variable or symbol macro named by the same symbol).

Any use of setq to set the value of Per X3J13. -kmp 5-Oct-93 one ofthe symbol while in the scope of this definition is treated as if it were a setf. psetq of symbol is treated as if it were a psetf, and multiple-value-setq is treated as if it were a setf of values.

A binding for a symbol macro can be shadowed₂ by let or symbol-macrolet.

Examples:

(defvar *things* (list 'alpha 'beta 'gamma)) → *THINGS*

(define-symbol-macro thing1 (first *things*)) → THING1
(define-symbol-macro thing2 (second *things*)) → THING2
(define-symbol-macro thing3 (third *things*)) → THING3

thing1 → ALPHA
(setq thing1 'ONE) → ONE
*things* → (ONE BETA GAMMA)
(multiple-value-setq (thing2 thing3) (values 'two 'three)) → TWO
thing3 → THREE
*things* → (ONE TWO THREE)

(list thing2 (let ((thing2 2)) thing2)) → (TWO 2)

Affected By:

None.

Exceptional Situations:

See issues LISP-SYMBOL-REDEFINITION and SYMBOL-MACROS-AND-PROCLAIMED-SPECIALS.If symbol is already defined as a global variable, an error of type program-error is signaled.

See Also:

symbol-macrolet, macroexpand

Notes:

None.