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From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Tue, 2 Jun 2015 09:09:49 -0600
Subject: Re: Everything is an object in python - object class and type class
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On Mon, Jun 1, 2015 at 4:59 PM, BartC <bc@freeuk.com> wrote:
> I'm developing a new language along the lines of Python, perhaps a brief
> description of how things are done there might help. Or just give a
> different perspective.
>
> Objects in this language are tagged: there's a code attached to each
> indicating what kind of data is stored. These codes are integers, or
> enumerations, for example:
>
>  Int =    1
>  String = 2
>  List =   3
>  Class =  4
>
> And the following are examples of object instances with their tags:
>
> a is (Int,    42)
> b is (String, "ABCXYZ")
> c is (List,   [10,20,30,40])
> d is (Class,  2 or <String>)
>
> The last one might be tricky to grasp: it's really just a number, but one
> that represents a class or type (or tag). If printed, it could display
> <String> rather than just 2. (And when used to do something with the class,
> the 2 might be an index into a set of tables.)
>
> d is not /the/ class itself, but just a reference to it (this is pseudo-code
> not Python):
>
> print (b)                 =>  ABCXYZ
> print (typeof(b))         =>  String <2>
>
> print (d)                 =>  String <2>
> print (typeof(d))         =>  Class  <4>
> print (typeof(typeof(d))) =>  Class  <4>
>
> In my own language, the connection between class and type is hazy (I've only
> just added classes so it needs to be hazy until I understand them more).
>
> 'Type' includes everything, including all the built-in types such as the
> tags above, but also user-defined classes. In fact classes are the mechanism
> used to define new types. But with both designated by an integer within the
> same band (eg. built-int types 1 to 20, user-defined classes 21 and up), it
> is easier not to have a strong distinction ... at the moment.
>
> I haven't a root class yet that is the base of all the others. I don't think
> it's necessary internally to make things work. But it might be a useful
> concept in the language. Calling it 'object' however might give rise to
> confusion as 'object' is informally used to refer to instances. (I've never
> used OO but have picked up some of the jargon!)
>
> (This is almost certainly not how Python does things. Although the Python
> language doesn't go into details as to its implementation provided the
> behaviour is correct.)

It sounds quite similar to me. In CPython at least, every object has a
direct pointer to its type rather than an array index (which is
essentially just an offset from a pointer).