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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 63 |
1 files changed, 63 insertions, 0 deletions
@@ -25772,6 +25772,69 @@ and .code a respectively. +.coNP Functions @ lexical-var-p and @ lexical-fun-p +.synb +.mets (lexical-var-p < env << form) +.mets (lexical-fun-p < env << form) +.syne +.desc +These two functions are useful to macro writers. They are intended +to be called from the bodies of macro expanders, such as the bodies of +.code defmacro +or +.code macrolet +forms. The +.meta env +argument is a macro-time environment, which is available to macros +via the special +.code :env +parameter. Using these functions, a macro can enquire whether +a given +.meta form +is a symbol which has a variable binding or a function binding +in the lexical environment. +This information is known during macro expansion. The macro expander +recognizes lexical function and variable bindings, because these +bindings can shadow macros. + +.TP* Example: + +.cblk + ;; + ;; this macro replaces itself with :lexical-var if its + ;; argument is a lexical variable, :lexical-fun if + ;; its argument is a lexical function, or with + ;; :not-lex-fun-var if neither is the case. + ;; + (defmacro classify (sym :env e) + (cond + ((lexical-var-p e expr) :lexical-var) + ((lexical-fun-p e expr) :lexical-fun) + (t :not-lex-fun-var))) + + ;; + ;; This returns: + ;; + ;; (:lexical-var :not-lex-fun-var :lexical-fun) + ;; + (let ((x 1) (y 2)) + (symacrolet ((y x)) + (flet ((f () (+ 2 2))) + (list (classify x) (classify y) (classify z))))) +.cble + +.TP* Note: + +These functions do not call +.code macroexpand +on the form. In most cases, it is necessary for the macro writers +to do so. Not that in the above example, symbol +.code y +is classified as neither a lexical function nor variable. +However, it can be macro-expanded to +.code x +which is a lexical variable. + .coNP Operator @ defsymacro .synb .mets (defsymacro < sym << form ) |