Hi all,
Greeting from me!
When studying define-macro(//http://www.newlisp.org/downloads/newlisp_manual.html#define-macro), I am a little confused with following statement:
Quote
Note that in fexprs, the danger exists of passing a parameter with the same variable name as used in the define-macro definition. In this case, the fexpr's internal variable would end up receiving nil instead of the intended value:
;; not a good definition!
(define-macro (my-setq x y) (set x (eval y)))
;; symbol name clash for x
(my-setq x 123) → 123
x → nil
Could anyone elaborate why the fexpr's internal variable's name couldn't be same as parameter's name?
Thanks very much in advance!
Best Regards
Nan Xiao
Isn't it just a normal effect of dynamic binding; that any assignment always affects the "inner-most" variable of the name. It's not special to fexprs, but you can confuse yourself quite easily with "normal" lambdas if/when a lambda body refers to a variable outside its scope.
For example, if you have the following definitions (well, you wouldn't, but you can imagine it):
(define (myset x v) (mysetx v))
"hello"
> x
"hello"
> (myset 'x 123)
123
> x
"hello"
Clear as mud :)
Hi Ralph,
Got it! Thanks for your reply!
Best Regards
Nan Xiao