From: Glenn Parker <glenn.parker@...>
Date: 2005-01-20T06:57:37+09:00
Subject: Re: value by reference

David A. Black wrote:
> 
> On Wed, 19 Jan 2005, Glenn Parker wrote:
> 
>>  def inc1(a, i); a += i; end
>>  def inc2(a, i); a = a + i; end
>>
> In any case, I believe the two things in your example are actually
> equivalent.  My understanding is that Ruby always expands:
>    x += y
> to
>    x = x + y


Ouch!  That's what I get for posting code without trying it out first. 
Even something that trivial tripped me up, and I should know better.  :(


Austin Ziegler wrote:
> "a += i" is syntax magic for "a = a + i". Perhaps you're referring
> to something like:
> 
>   def app1(a, i); a << i; end
>   def app2(a, i); a += i; end


Right, that is closer to what I was trying to illustrate.  As to whether 
those two should be considered equivalent, I assert each person's answer 
will depend on their expectations and their prior experience.  :)


Austin Ziegler wrote:
> Except that Ruby's call model isn't that far from the way that Perl
> works. Probably not far from Python's model, either.

Perl generally uses call-by-value, but it also supports 
call-by-reference using the backslash operator:

   sub inc {
     $arg = shift @_;
     $$arg += 1;
   }
   $x = 1;
   inc(\$x);
   print $x;

This prints "2".  Here, "\$x" is a reference to $x, and "inc" assigns it 
to $arg.  $$arg dereferences the reference, allowing inc to modify $x 
directly.

Like Ruby, Python doesn't seem to have any support for 
call-by-reference, and Python coders have been through the exact same 
discussion that we're having here (almost word-for-word): 
http://mail.python.org/pipermail/python-list/2001-June/048603.html


Florian Gross wrote:
>> the Tcl core has the "uplevel" and "upvar" builtins.
> 
> I'm not experienced at Tcl. What do those built ins do? 

Tcl's "upvar" is a generalized call-by-reference method.  It allows a 
function to bind a local variable to variable from the calling scope.

   proc inc name {
     upvar $name a
     set a [expr $a + 1]
   }
   set x 1
   inc x
   puts $x

This also prints "2".  In this case, "a" becomes an alias for "x" after 
the call to upvar.  Tcl's "uplevel" is very similar in concept to 
Binding.of_caller, allowing execution of code in the caller's scope. 
Tcl coders can use uplevel to create new flow-of-control structures.

I've seen one or two posts here that flat out denigrate 
call-by-reference, or suggest that it is a workaround necessitated by 
weak language design.  This is a narrow view, IMHO.  If we can define 
methods that modify an object in place, then we are already halfway to 
using call-by-reference semantics.

Oh well, this was probably more educational for me than for anybody 
else, so thanks to everybody for your patience.

-- 
Glenn Parker | glenn.parker-AT-comcast.net | <http://www.tetrafoil.com/>