Re: Side-Effects in Functions [Rosen Trick]

["Warren W. Gay VE3WWG" <ve3wwg@home.com>]

Dmitry A. Kazakov wrote:

> On Mon, 5 Nov 2001 15:21:13 +0000 (UTC), Preben Randhol
> <randhol+abuse@pvv.org> wrote:
>>On Mon, 05 Nov 2001 14:59:15 GMT, Dmitry A. Kazakov wrote:
>>>On 5 Nov 2001 03:19:39 -0800, pete@nospam <pete_member@newsguy.com>
>>>wrote:
...
> Yes. But I still have a choice to make it:
> 
> procedure Find (Folder : in out Table; Result : out Table_Token; 
>                 Unique : out Boolean);
> 
> It is of course not so that any out parameter has to be declared as
> the result. All we need is an impure function. As Robert have pointed,
> it is ridiculous to have an ability to write impure functions and have
> no right to manifest them as such (openly using in out parameters). If
> the word "function" makes people so sensitive, let's use "procedure"
> instead!
> 
>>What is the difference between a function with in out and a procedure
>>with a return value? It amounts to the same thing I think.
> 
> There is no difference other than functions with in out parameters are
> unacceptable for some part of Ada community, which is enough powerful
> to ban them forever (:-)).
> 
> P.S. I do not belong to this part.
> 
> Regards,
> Dmitry Kazakov


I think one of the issues at stake here is that Ada was meant to be

easy to read. If you allow procedures to return values, then it is
no longer a valid assumption that what looks like a function call,
does not produce side effects (ie. you cannot assume that the arguments
are not modified, without looking at the declaration).


To pull it off, you'd have to make some sort of syntactical difference

in procedure-function calls, to maintain the status quo for functions.
In case I am not explaining this clear enough, the following example
might help:

declare
    Arg : Boolean;		-- Argument
    R   : Boolean;		-- Result
begin

    R := My_Function(Arg);	-- Assumed that Arg is not modified
    R := My_Proc_Function[Arg];	-- A procedure returning.. identified by []


I am not suggesting the [] make a good syntax here, but you'd have
to do something like this to maintain the distinction that Functions
already enjoy within Ada.
-- 
Warren W. Gay VE3WWG
http://home.cogeco.ca/~ve3wwg