From: Brian Candler <B.Candler@...>
Date: 2002-11-05T20:18:47+09:00
Subject: Re: Thoughts on Ruby

On Mon, Nov 04, 2002 at 10:02:12PM +0900, Gavin Sinclair wrote:

(an all-encompassing statement if ever I saw one :-)

> a += b is always, by definition, in any language, the same *behaviour* as a = a
> + b.  If you disagree, you are being absurd.

By whose definition? Certainly not the language designers and standards
bodies. As a counter-example: in C++, if an object has both "+" and "+="
methods, the language definition does not require them to have the same
behaviour, and therefore in general they don't. QED.

In those languages, "+=" is just a label ( / token / method name).

There may be some sort of shared understanding between certain groups of
programmers that "+" and "+=" ought to work in the way you describe, but
it's not a "definition". Programmers in certain languages may reasonably
assume that a += b modifies the object referred to by 'a' in place, which is
not the behaviour you describe. It's just part of the conventions of that
particular language.

Actually, many languages don't even _have_ a "+=" operator, which also
breaks your statement. However I will assume "any language" is replaced with
"any language which has these operators" in the above :-)

> The only possible difference is
> in efficiency: run-time hacks that save an intermediate object.  This is a
> sometimes a valid concern, and there is a remedy: the << operator.
> 
>   a = a + b       (old object 'a' lost)
>   a << b          (same behaviour, *possibly* more efficient)
> 
> This behaviour is defined for Strings and Arrays

Those two examples do not exhibit the same behaviour in the presence of
other variables containing references to the same object as the original
'a'. That's not a "run-time hack", that's a difference in semantics: i.e. "I
want to create a new object and leave the old one alone", as opposed to "I
want to modify the object itself". The difference is important and not just
one of efficiency, unless we know for definite that there are no such
additional references.

I don't think we should restrict ourselves to Strings and Arrays, since you
made a sweeping statement about operators in general. Even if there is a
shared understanding of what we mean by "+" or "+=" on a String, it might
not be so clear for a Foo object. Furthermore, the "<<" example is
meaningless where 'a' references an immutable object; it may calculate some
value, but can't modify the object in place.

Anyway, I think the point I'm trying to make is:

1. There are two valid ways to apply an operator and argument to an object:
   create a new object, or modify in place (if it is mutable)
2. You need two different operators to be able to distinguish them
3. Whether they are called "+" and "+=" (as a C++ programmer might use),
   or "+" and "<<" (as a Ruby programmer would use, since he doesn't have
   the option of giving different semantics to "+="), or "foo" and "foo!",
   is pretty irrelevant.

It's just convention. To me, coming from a C background, it looks odd that
"integer left shift by X" also means by convention "append value X", but you
get used to it as part of learning the language.

Regards,

Brian.