[self-interest] Re: data structures

Fri Jun 11 12:09:37 UTC 1999

Huh? I didn't understand... See my comments below.
Regards,
Douglas

David.Ungar at Eng.Sun.COM wrote:

> Amen!
>
> When designing Self
> I came to believe (and still do) that the idea of 'nil' is not
> object-oriented at all.

Why isn't it object-oriented? I've always understood nil (or NULL, or null) as
"nothing", "no object". If I say "What object is pointed to by this slot?" and the
answer is "no one" then it seems right that nil is there. I thought nil was
necessary to have some value or some object to indicate this absence. In C++, for
instance, the conceptual nil object (which is at adress 0, aliased NULL, right?)
exists as some object whose address is 0x00000000. If the implementation cares
about it (and I think they all must care) there should be no real object or
variable at address 0, so that no real object can have the same address as NULL.
Then, why not think the memory at address 0 contains an object? I am used to fake
to myself that there is a system-defined object there named NULL, whose address is
0 (but could be any other address) and it seems conceptually clean. Many algorithms
I have implemented take into account that a pointer containing NULL refers to
"nothing", meaning that none of the objects that interest the algorithm is there.
If we come to the implementation side (pardon me again :-) we can't avoid to
consider that every reference to an object is implemented as a pointer, that is, as
a memory location whose value is an address. So how do you say "This slot doesn't
point to anything" ? Leave it with a random value? This seems dangerous, as you can
always coincidently refer to an existing object, or to garbage (which can be part
of an object's memory). Don't we need the concept of "no object"? Could you please
explain this again?

>
> It is just a holdover from Lisp via Smalltalk.
> If I had to do it all over again, there would be no nil in Self.
>
> (So why? Because 'nil' is the same no matter what kind of object
> isn't there. This leads to code such as:
>
> nil = foo ifFalse: [
>     <do something with foo>
> ]
>
> If instead, you make a special object that respects the right
> protocol you could write
>
> foo doSomething
>
> or
>
> foo doSomethingIfNotInitialized: [...]
>
> - Dave
>
> PS: I'm not sure my messages are reaching the alias--could someone verify?

You mean self-interest at egroups.com? I got your message from there.

>
>
> At 3:48 AM -0300 6/9/99, Jecel Assumpcao Jr wrote:
> >Ian Trudel wrote:
> >> : In Smalltalk we would do "vector new: 3" since everything is
> >> : initialized with nil, but Self doesn't like defaults and so you
> >> : have to choose some initial value (like 0 in this example)
> >> : explicitly.
> >>
> >> Out of interest, why Self doesn't like defaults? Time consuming? Personnaly,
> >> I prefer everything has a default value before uses. Maybe just because I
> >> am, and every programmer I know, are human and sometimes forgot to
> >> initialize.
> >
> >Self is based on prototypes instead of classes. A class includes,
> >among many other things, a plan to build object instances of itself.
> >This plan (the #new method in Smalltalk, the constructor in other
> >languages) will normally replace any defaults that the system uses
> >when allocating the instance with values that are appropriate for
> >that class.
> >
> >A prototype, on the other hand, is usually "hand made". It can be
> >cloned to generate new "instances", but it is an actual working
> >"instance" itself and its slot values should reflect this. We could
> >have a Record class in Smalltalk with this creation method:
> >
> >   new
> >   "creates an initialized Record object"
> >   ^ self basicNew initialize
> >
> >where the instance method is:
> >
> >   initialize
> >   "sets the initial values for the Record object"
> >   name := 'unnamed'.
> >   age := 0.
> >   ^ self
> >
> >We could do the exact same thing in Self, of course:
> >
> >   (| parent* = traits clonable.
> >      name.
> >      age.
> >      copy = ("creates a new initialized record"
> >              resend.copy initialize).
> >      initialize = ("sets the initial values"
> >                    name: 'unnamed'.
> >                    age: 0.
> >                    self)
> >   |)
> >
> >but a the following would be much more Selfish:
> >
> >   (| parent* = traits clonable.
> >      name <- 'unnamed'.
> >      age <- 0
> >   |)
> >
> >In the first Self example, the slot values for the prototype are
> >the default (nil). They are a string and an integer for every
> >object clones from the prototype, however, so the prototype is not
> >a true representative of this kind of object. The second example
> >fixes this, but is now subject to the "prototype corruption
> >problem". That is when you change the prototype itself instead of
> >a copy (by sending it the message 'age: 10', for example). This
> >will effect every new clone created in the future.
> >
> >So what I should have said in my comment what that in Self we should
> >always avoid depending on defaults like nil but should instead create
> >the prototypes with the right types of values (which are another kind
> >of defaults, of course).
> >
> >-- Jecel
> >
> >
> >------------------------------------------------------------------------
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>
>
>      David Ungar
>      Sun Microsystems Laboratories
>      (650) 336-2618
>
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