Returning by reference

[kks]

Hi,



How to "return by reference" from a user-defined function/macro?

Code Select Expand


(let (l '(a b c)) (pop l) l)   # => (b c)
(let (l '(a b c)) (pop (or l)) l)   # => (b c)
(let (l '(a b c)) (pop (println l)) l)   # => (a b c); is this a bug?
(let (l '(a b c)) (pop (begin (println l) l)) l)   # => (b c)
(let (l '(a b c)) (pop (let () l)) l)   # => (a b c); is this a bug?
(let (l '(a b c)) (pop ((fn () l))) l)   # => (a b c); how to make it return (b c)?


My intention is to write a debugging macro:

Code Select Expand


(define-macro (%p) (println "% " (args 0) " = " (eval (args 0))))

(let (l '(a b c)) (pop (%p l)) l)   # => (a b c); how to make it return (b c)?


Thanks,

KS

[kks]

By the way, in the following code:

Code Select Expand


(setq x ((fn () '(a b c))))   # => (a b c)

does it perform ONE (either when returning or binding) or TWO (both) copy operations?



Is it possible to make all sequences return "by reference"? I think this still conforms to ORO (One Reference Only) memory management, as it is "binding" (set, let, parameter binding), not "returning", that causes "referencing"!

[cormullion]

(let (l '(a b c)) (pop (println l)) l)   # => (a b c); is this a bug?

Hi! I would guess not a bug:

Code Select Expand

> (set 'l '(a b c))
(a b c)
> (println l)
(a b c)
(a b c)
> (pop (println l))
(a b c)
a
> l
(a b c)


since pop operated on the result returned by println rather than on l. That's only my guess, though...

[itistoday]

User defined functions always pass things around by value, but there are ways of getting around this by passing "containers" by value (i.e. symbols or contexts).



You may want to look at the thread on the http://newlispfanclub.alh.net/forum/viewtopic.php?f=16&t=3413">gen-wrapper function.