Elisp info request: function parameter passing

Elisp info request: function parameter passing

[richardeng]

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I just found elisp is different in function parameter passing than C, or 
we can say elisp is reference passing style. That is, the variable in 
function call is itself, the same one in elisp language. Following code 
can show this different feature.

(setq x '(1 2 3 "one" "two" "three"))
(defun fun (arg)
   (if (eq arg x)
       (message "they are eq, the same one")
     (message "they are different")))
(fun x)

I think we need to add more info about it in (elisp)Top > Functions

Another question:
I think change the element of list should be a common request. Why 
doesn't elisp provide a primitive/function for this feature. Did I miss it?

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From: richardeng <richardeng@foxmail.com>
To: Alan Mackenzie <acm@muc.de>
Cc: help-gnu-emacs@gnu.org
Subject: Re: How could I modify list element?
Date: Wed, 10 Dec 2008 01:43:23 +0800
Message-ID: <493EAE3B.8020707@foxmail.com>

Alan Mackenzie wrote:
> 
> 
> On Tue, 9 Dec 2008, richardeng wrote:
> 
>> Hi all,
>>    setcar/setcdr is not convenient.
>>    In a long list, ex.
>>    (setq a '(a b c d e f g))
>>    I want to change 'e to 'E.
>>    I need a function: (set-list-elt list old-elt new-elt)
>>
>>    How? translate list to vector, modify, then turn it back???
> 
> Try this (not tested):
> 
> (defun change-nth (liszt, n, nieuw)
>   (while (> n 0)
>     (setq liszt (cdr liszt)
>           n (1- n)))
>   (if liszt
>      (setcar liszt nieuw)))
> 
(setq aaa '(1 2 3 4 5 b a "ccc"))
(change-nth aaa 3 'BB)
aaa  --> (1 2 3 BB 5 b a "ccc")    , it works, thank you!

Re: Elisp info request: function parameter passing

[richardeng]

richardeng wrote:
> I just found elisp is different in function parameter passing than C, or 
> we can say elisp is reference passing style. That is, the variable in 
> function call is itself, the same one in elisp language. Following code 
> can show this different feature.
> 
> (setq x '(1 2 3 "one" "two" "three"))
> (defun fun (arg)
>   (if (eq arg x)
>       (message "they are eq, the same one")
>     (message "they are different")))
> (fun x)
> 
> I think we need to add more info about it in (elisp)Top > Functions
> 
> Another question:
> I think change the element of list should be a common request. Why 
> doesn't elisp provide a primitive/function for this feature. Did I miss it?
> 
> ------------------------------------------------------------------------
> 
> Subject:
> Re: How could I modify list element?
> From:
> richardeng <richardeng@foxmail.com>
> Date:
> Wed, 10 Dec 2008 01:43:23 +0800
> To:
> Alan Mackenzie <acm@muc.de>
> 
> To:
> Alan Mackenzie <acm@muc.de>
> CC:
> help-gnu-emacs@gnu.org
> 
> 
> Alan Mackenzie wrote:
>>
>>
>> On Tue, 9 Dec 2008, richardeng wrote:
>>
>>> Hi all,
>>>    setcar/setcdr is not convenient.
>>>    In a long list, ex.
>>>    (setq a '(a b c d e f g))
>>>    I want to change 'e to 'E.
>>>    I need a function: (set-list-elt list old-elt new-elt)
>>>
>>>    How? translate list to vector, modify, then turn it back???
>>
>> Try this (not tested):
>>
>> (defun change-nth (liszt, n, nieuw)
>>   (while (> n 0)
>>     (setq liszt (cdr liszt)
>>           n (1- n)))
>>   (if liszt
>>      (setcar liszt nieuw)))
>>
> (setq aaa '(1 2 3 4 5 b a "ccc"))
> (change-nth aaa 3 'BB)
> aaa  --> (1 2 3 BB 5 b a "ccc")    , it works, thank you!
> 

Additional comment for C programmer:

In C, when you change the value of variable you are changing the 
variable's value, the old value is lost.

In elisp, when you change the value of variable you are changing the 
target of the variable, the old value remains. Old value will be GC 
until zero reference

So, when you pass a variable to a function. The parameter is a new 
variable but point to the same value. If you change the parameter in 
function, you just change the target of the parameter variable. Unless, 
you use setcar/setcdr/etc. to change the value cons. Note, many other 
functions will copy the value.

Lisp is really beautiful and special in design.

Re: Elisp info request: function parameter passing

[Stefan Monnier]

> I just found elisp is different in function parameter passing than C, or we
> can say elisp is reference passing style. That is, the variable in function

You are confused.

> (setq x '(1 2 3 "one" "two" "three"))
> (defun fun (arg)
>   (if (eq arg x)
>       (message "they are eq, the same one")
>     (message "they are different")))
> (fun x)


Try the equivalent C code:

  char *x = "hello";

  fun (char *arg)
  {
    if (arg == x)
      printf ("they are eq\n");
    else
      printf ("they are different\n");
  }

  main ()
  {
    fun (x);
  }


-- Stefan