[Axiom-developer] functional type system using pointer comparisons to check equality [was: Re: [Aldor-l] [Axiom-math] Are Fraction and Complex domains.]

[Christian Aistleitner]

Hello,

On Tue, 16 May 2006 15:08:16 +0200, Ralf Hemmecke <address@hidden> wrote:

> > DomA( a: List Integer ):with {...} == add {...}
> >  a: List Integer := [ 1, 2, 3 ];
> > R == DomA( a );
> > a . 2 := 300;
> > S == DomA( a );
> >  a still refers to the same memory location, but it is different. Would  
> > R equal S?
>
> Well, here I would comment the following.
>
> a.2 := 300
>
> is the same as
>
> set!(a, 2, 300)
>
> Everyone sees the exclamation mark? This exclamation mark is a  
> _convention_ in Aldor to say that a function is destructive,

Yes.

> ie it destroys one of its parameters.

No. It may destroy _all_ of its parameters. But thats not the point.

Since it is hard -- or rather impossible -- for a developer to deceide  
whether or not a value he returns will be or has been used in a type  
constructing function, you'd practically forbid using destructive  
functions at all.
You'd not even be allowed to act destructivly on a value you just copied:

SomeDom: with {
  CopyableType;
  ...
} == add {
  copy( a: % ): % == {
    local copiedValue: %;
    ...
    copiedValue := ...;
    ...
    assert( hasBeenProperlyCopied $ ProperlyCopiedHelperDomain( a,  
copiedValue ) );
    copiedValue;
  }
  ...
}

local a: SomeDom := ...
local b: SomeDom := copy a;
destructiveFunction!( b ); -- b has already been used in via copiedValue  
in ProperlyCopiedHelperDomain( a, copiedValue ).

So practically no destructive functions...
I wouldn't want to have a functional-type system performing equality  
checks by comparing pointers.
Besides, huge Integers representing the same number most likely do not  
share the same memory.
Besides, Lists of the same integers most likely do not share the same  
memory.
Most "elements of domains", which represent the same value most likely do  
not share memory. However, they can be used as parameters to domain  
constructing functions. What would be the worth of "functionality" if it  
stops there?

--
Kind regards,
Christian