Re: ANN: Seed7 Release 2008-02-17

On 18 Feb., 22:02, Bart <b...@[EMAIL PROTECTED]
> wrote:
> On Feb 17, 10:13 pm, thomas.mer...@[EMAIL PROTECTED]
 wrote:
>
> [From Seed 7 FAQ]
>
> >Variables with object types contain references to object values....
> > a := b
> >For primitive types a different logic is used...
> > a := b
> >both variables are still distinct and changing one variable has no
effect on the other.
>
> This is more of a general question about a:=b for a class object being
> different from a:=b for an ordinary object. It seems to be the case in
> several oo languages.
>
> Why?
IMHO it is a cornerstone of classic OO programming languages
like Smalltalk and its descendants: Object variables and
parameters contain just pointers to the actual object value
which is at the heap. It is almost not mentioned by OO
proponents but in classic OO programming not everything is
done with methods. Assignments are always done as pointer
assignments, independend of the object class. Also comparisons
are done as pointer comparisons. Therefore a 'clone' method
to do deep copies and an 'compare' method to do logical
comparisons is necessary.

The advantage of this OO pointer philosophy is that container
classes are easier to implement (they store just pointers in
the container).

> OK, objects are implemented by reference and all that, and maybe this
> saves some copying, but that should all be transparent.
>
> Surely the meaning of something as basic as a:=b should not depend on
> something as arbitrary as whether a and b have datatypes T or U. If
> someone changes their mind then a lot of recoding might be needed!

Most OO programmers have the different assignment logic for
object variables in their mind. They would be confused when
those assignments would work as deep copies (As Seed7 does
for all other types).

IIRC Simula had two different assignment operators:
  := For value copy assignments
  :- For reference assignments

Seed7 is an extensible programming language.
It would be possible to define the assignment to do a (deep)
copy of the implementation (object) value and to have
a different operator to do reference assignment.

I don't belive that OO fan's would be happy without
reference assignment?

What do you think?

Greetings Thomas Mertes

Seed7 Homepage:  http://seed7.sourceforge.net
Seed7 - The extensible programming language: User defined statements
and operators, abstract data types, templates without special
syntax, OO with interfaces and multiple dispatch.