[ clisp-Bugs-3451264 ] odd error with sort

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Bugs item #3451264, was opened at 2011-12-05 03:40
Message generated for change (Comment added) made by sds
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Category: clisp
Group: lisp error
>Status: Pending
>Resolution: Invalid
Priority: 5
Private: No
Submitted By: Vladimir Lidovski (litwr)
>Assigned to: Sam Steingold (sds)
Summary: odd error with sort

Initial Comment:
It is about GNU CLISP strange behaviours. The version 2.33/2.48/2.49 were tested. Can anybody explain this oddness?

(defun xsort (l) (sort l #'<)) ;XSORT
(setq l '(3 1 2)) ;(3 1 2)
(xsort l) ;(1 2 3)
l ; surprize! (1 2 3)

The ordinal lisp function XSORT takes its argument by value but the presence of SORT-function in its body converts argument to passed by reference! Are there any reasonable explanation? BTW the POP-function doesn't make surprize:

(defun xpop (l) (pop l)) ;XPOP
(setq l '(3 1 2)) ;(3 1 2)
(xpop l) ;3
l ;(3 2 1)


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>Comment By: Sam Steingold (sds)
Date: 2011-12-05 08:43

Message:
This is the way lisp works.
sort is a destructive function, it modifies the list structure.
other CL implementations use different modifications, so l might not be the
sorted list.
however, both behaviors are standard-compliant.
try this:
(setq m l)
(setq s (xsort l))
(eq m l) ==> t, because xsort cannot modify l
(eq s l) ==> t, because that's how clisp works

pop is a macro, so it modifies the its argument's value.
xpop is a function, so it cannot modify the value of l.

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Comment By: Sam Steingold (sds)
Date: 2011-12-05 08:43

Message:
This bug report is now marked as "pending"/"invalid".
This means that we think that the problem you report is not a problem with
CLISP.
Unless you - the reporter - act within 2 weeks, the bug will be permanently
closed.
Sorry about the inconvenience - we hope your silence means that you agree
that this is not a bug in CLISP.

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You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=101355&aid=3451264&group_id=1355