Re: How do you choose your programming language ?

On Wed, Nov 23, 2005, Zohar Kelrich wrote about "Re: [hackers-il] How do you 
choose your programming language ?":
> Actually, the Java spec says nothing whatsoever about what sort of  
> implementation the GC should have; its only GC "contract" is, IIRC,  
> no object gets finalized more than once.
> You're free to free (as it were)  objects at the end of every method,  
> if you so choose.

Just like understanding human languages requires several levels (syntax,
semantics, pragmatics, and world knowledge), so does understanding a
programming language. You cannot realisticly seperate a language and its
syntax from the language's standard library, and accepted practices.
Also, the syntax and semantics it provides, or doesn't provide, implies on
the way people use it, and therefore on the way typical programs are written
in this language.

In this example, one very fundamental point about Java is that there is no
free() call and no objects that live only for the duration of a single
scoped (such as "automatic" (on-stack) variables in C/C++, or "autoptr" in
C++). So generally, a user cannot do what you describe in Java (he has no
free() function), and every Java implementation, whether an "interpreter" or
"compiler", CANNOT do what you describe in the general case. Because,
imagine the following piece:

        {
                Thing a = new Thing(17);
                counter+=someobject.doSomethingWith(a);
        }

Can a be freed automatically right after this block (by an interpreter or
a compiler, it doesn't matter)? It's not easy to decide. It depends on
what doSomethingWith() does. Typically, what this method does is not known
during compilation of the individual classes. Also, someobject might be of
different classes (because of inheritence) and each of these classes' method
might do something else. But even if you know exactly which method is called,
it's still not easy to know if a pointer to a is kept. Perhaps that method
calls other methods with a, or with parts of a as parameters, and these
methods saves pointers?

Don't understand me wrong - the very fact that everything in Java is a
reference, and there're no error-prone pointers/autopointers/automatic
variables/malloc-free, and no chance to accidentally use an out-of-date
pointer or forget to free one - is what makes Java different, and easier to
write than languages like C++. It's a deliberate design decision, and not
an implementation mistake.  But at the same time, it's what makes it a
memory hog.

-- 
Nadav Har'El                        |  Wednesday, Nov 23 2005, 22 Heshvan 5766
nyh-TS7m/3hpY0sOpacJJkBjfT4kX+cae0hd@xxxxxxxxxxxxxxxx             
|-----------------------------------------
Phone +972-523-790466, ICQ 13349191 |My password is my dog's name. His name is
http://nadav.harel.org.il           |a#j!4@h, but I change it every month.


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