Destructive functions?

[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

[cormullion]

In that example, move is local to the function, and won't survive the function.

Obviously, this is one way:

Code: Select all

(set 'm (strip m))

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...

[Jeff]

You don't need a macro for that; just pass a quoted symbol.

[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