>>>>> Hans Nowak <[EMAIL PROTECTED]> writes:
> #;1> (define magic 42) #;2> (define s '(foo bar ,magic))
[...]
> The following doesn't work:
> #;4> `s s #;5> `,s (foo bar (unquote magic))
> ...and I understand *why* they don't work, but I can't figure
> out how to take s and transform it into (foo bar 42).
quasiquote is syntax - it happens at compile-time, so to
apply it to dynamic data at runtime you need to call EVAL:
(eval (list 'quasiquote s))
; => (foo bar 42)
EVAL may be overkill for something this simple - an
alternative would be to write a simple substitution
function:
(use lolevel)
(define (replace-unquotes x)
(if (not (pair? x))
x
(if (and (pair? (cdr x)) (null? (cddr x)) (eq? 'unquote (car x)))
(global-ref (cadr x))
(cons (replace-unquotes (car x))
(replace-unquotes (cdr x))))))
(replace-unquotes s)
; => (foo bar 42)
You'd of course have to extend this if you want to handle
nested quasiquotes and unquote-splicing.
--
Alex
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