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I don't understand the 3 line explanation of backquotes in backquotes
on p. 350 of CLtL. I'd appreciate some enlightenment. Also, I'd be
interested in whether backquote is capable of solving the problem below.
I surely don't see how to do it, but since I don't really understand
embedded backquotes I'm not sure it can't be done.
Suppose we want to declare versions of existing functions that do type
checking, e.g., we want a protected version of (+ x y) which, if x and y
are both numbers returns their sum and otherwise returns nil. This could
be done by the following macro:
(defmacro protected-+ (x y)
`(let ((x ,x) (y ,y)) (and (numberp x) (numberp y) (+ x y))))
(macroexpand '(protected-+ a b))
(LET ((X A) (Y B))
(AND (NUMBERP X) (NUMBERP Y) (+ X Y)))
(The let assures that each argument is evaluated only once.)
Since there are many such functions to declare, we might want a macro
to define them, e.g.,
(define-protected + (numberp x) (numberp y))
would be a reasonable way to create the macro above.
If we try to define this macro in the straight forward way we start
out like so:
(defmacro define-protected (function &rest args)
`(defmacro ,(MAKE-PROTECTED-NAME function)
,(mapcar #'cadr args)
`(let ,(BUILD-LET-LIST args)
,(BUILD-FUNCTION-CALL function args)))))
Unfortunately, it appears that BUILD-LET-LIST has to build an expression
with more commas than backquotes, which the backquote readmacro cannot do.
Another way to look at it is that I don't see how one backquote can
produce different backquoted expressions with varying numbers of commas.
- From: Barry Margolin <barmar@Think.COM>
- From: Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>
- From: Christopher Garrigues <7thSon@SPAR.SLB.COM>
- Re: backquote
- From: "Stephen P. Smith" <email@example.com>