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Destructive functions?
Posted: Mon Apr 14, 2008 9:08 pm
by hsmyers
What is the pattern for writing functions that change their parameters in the calling scope? For instance this clearly doesn't work:
Code: Select all
(define (strip move)
(replace "[+#]" move "" 0))
So what would?
--hsm
Posted: Mon Apr 14, 2008 10:01 pm
by cormullion
In that example,
move is local to the function, and won't survive the function.
Obviously, this is one way:
cos you're usually working with the values of evaluated functions. But how about this:
Code: Select all
(define (strip move)
(replace "[+#]" move "" 0))
(set 'm "+blah")
(strip m)
;-> "blah"
m
;-> "+blah"
(define-macro (strip move)
(replace "[+#]" (eval move) "" 0))
(set 'm "+blah")
(strip m)
;-> "blah"
m
;-> "blah"
Looks like we can pass the symbol itself...
Posted: Mon Apr 14, 2008 11:46 pm
by Jeff
You don't need a macro for that; just pass a quoted symbol.
Posted: Tue Apr 15, 2008 12:33 am
by hsmyers
Thanks all. The use of eval means that I can use it either way, as a value for set or setq and as a direct change--- cool!
--hsm