# Meta-class subclass relationships

**URL:** <https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371>\
**Category:** Ruby\
**Created:** [November 16, 2007, 10:20am UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371 "2007-11-16T10:20:03Z")\
**Posts on this page:** 19\
**Page:** 1

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 10:20am UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/1 "2007-11-16T10:20:03Z")

</div>

Ruby exposes its singleton meta-classes, eg:

```
class <<B ; $Meta_B = self ; end

```

But their relationships are not quite what I expected. For example:

Suppose that A inherits from B (and B inherits from Object). Of course,  
this means that A instances respond to all the B instance messages. So:

```
A.new.is_a? B -> true

```

At the same time, the A class object responds to all the B class object  
messages. So:

```
class <<Object ; $Meta_Object = self ; end
class <<B ; $Meta_B = self ; end
class <<A ; $Meta_A = self ; end

A.is_a? $Meta_A -> true
B.is_a? $Meta_A -> false
A.is_a? $Meta_B -> true
B.is_a? $Meta_B -> true

```

Since everything that is\_a $Meta\_A also is\_a $Meta\_B, I expected a  
subclass/superclass relationship. Indeed, I expected the following to  
be true:

```
$Meta_A.superclass == $Meta_B
$Meta_B.superclass == $Meta_Object
$Meta_Object.superclass == Class
Class.superclass == Module
Module.superclass == Object
Object.superclass == nil

```

The above superclass chain reflects how a message to the class A object  
is looked up. What surprised me is that the first two equalities above  
are false.

Should they have been true?

FWIW: The superclass of both $Meta\_A and $Meta\_B is something called  
#Class:Class, which is its own superclass and is a subclass of  
$Meta\_Object:

```
$X = $Meta_A.superclass -> #<Class:Class>
$X == $Meta_B.superclass -> true
$X == $X.superclass -> true
$X < $Meta_Object -> true
```

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 10:41am UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/2 "2007-11-16T10:41:53Z")

</div>

PS: I just realized that my (erroneous) expectation

```
$Meta_A.superclass == $Meta_B
$Meta_B.superclass == $Meta_Object

```

is asserted in Figure 24.2 of the Pickaxe book (2nd edition). Eg, for  
the second equality, just replace “Guitar” with “B”. So why, I wonder,  
isn’t it true?

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 11:09am UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/3 "2007-11-16T11:09:06Z")

</div>

Hmm seems consistent for me, although the model you expect should have  
some merits, I believe Smalltalk satisfies your expectations…  
However there are no metaclasses in ruby, only singleton classes,  
naming is important here to emphasis on the difference in behavior.

635/135 \> irb  
irb(main):001:0\> a = Class::new  
=\> #Class:0xb7d89750  
irb(main):002:0\> ma = class \<\< a; self end  
=\> #\<Class:#Class:0xb7d89750\>  
irb(main):003:0\> b = Class::new a  
=\> #Class:0xb7d77500  
irb(main):004:0\> mb = class \<\< b; self end  
=\> #\<Class:#Class:0xb7d77500\>  
irb(main):005:0\> b \< a  
=\> true  
irb(main):006:0\> mb \< ma  
=\> nil  
irb(main):007:0\> mb.superclass  
=\> #Class:Class  
irb(main):008:0\> ma.send :define\_method, :a do 42 end  
=\> #Proc:0xb7d56724@:8(irb)  
irb(main):009:0\> ma.a  
NoMethodError: undefined method `a’ for #\<Class:#Class:0xb7d89750\>  
from (irb):9  
from :0

Stupid me, of course I defined a method for a

irb(main):010:0\> a.a  
=\> 42  
irb(main):011:0\> b.a  
=\> 42  
which works for b too, as b \< a

irb(main):012:0\> mma = class \<\< ma; self end  
=\> #\<Class:#\<Class:#Class:0xb7d89750\>\>  
irb(main):013:0\> mmb = class \<\< mb; self end  
=\> #\<Class:#\<Class:#Class:0xb7d77500\>\>  
irb(main):014:0\> mma.send :define\_method, :ma do 42 end  
=\> #Proc:0xb7d3e228@:14(irb)  
irb(main):015:0\> ma.ma  
=\> 42  
Now this was for ma  
irb(main):016:0\> mb.ma  
NoMethodError: undefined method `ma’ for #\<Class:#Class:0xb7d77500\>  
from (irb):16  
from :0  
and as mb \< ma does not hold this makes sense.

BTW meta caught me I should have called the guys above sa, sb, ssa and  
ssb ☹

Interesting that the Pickaxe got this wrong, never publish code you  
have not run ;), or maybe this changed between 1.6 and 1.8?

Cheers  
Robert

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 12:23pm UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/4 "2007-11-16T12:23:36Z")

</div>

Hi –

On Fri, 16 Nov 2007, pluskid wrote:

> > Posted viahttp://www.ruby-forum.com/.
> 
> That’s why you see “$Meta\_A.superclass → #Class:Class”. Maybe it  
> is not very useful. But it is the case now. And here are some links if  
> you are interested in:
> 
> - [http://whytheluckystiff.net/articles/seeingMetaclassesClearly.html](http://whytheluckystiff.net/articles/seeingMetaclassesClearly.html)
> - [http://practicalruby.blogspot.com/2007/02/ruby-metaprogramming-introduction.html](http://practicalruby.blogspot.com/2007/02/ruby-metaprogramming-introduction.html)

The superclass relationship between singleton classes is actually  
useful – it allows subclasses to call the singleton methods of their  
superclasses. (Which means that “singleton” is probably in some doubt  
as the best term in this case 🙂 I’m not sold on “meta”, though.)

Strictly speaking, this ability can be implemented with or without the  
superclass model; in fact, some recent Ruby versions will report false  
if you test for superclass-ness in these cases. But it’s been  
restored, and it makes a lot of sense as a way of fitting this  
otherwise kind of anomalous piece into the puzzle.

David

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 11:17am UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/5 "2007-11-16T11:17:54Z")

</div>

On 11ÔÂ16ÈÕ, ÏÂÎç5ʱ20·Ö, Greg W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> FWIW: The superclass of both $Meta\_A and $Meta\_B is something called  
> #Class:Class, which is its own superclass and is a subclass of  
> $Meta\_Object:
> 
> ```
> $X = $Meta_A.superclass -> #<Class:Class>
> $X == $Meta_B.superclass -> true
> $X == $X.superclass -> true
> $X < $Meta_Object -> true
> 
> ```
> 
> –  
> Posted viahttp://www.ruby-forum.com/.

In fact, currently in ruby:

A.metaclass.superclass == A.class.metaclass

if you define metaclass as :

class Object  
def metaclass; class \<\< self; self; end; end  
end

That’s why you see “$Meta\_A.superclass → #Class:Class”. Maybe it  
is not very useful. But it is the case now. And here are some links if  
you are interested in:

- [http://whytheluckystiff.net/articles/seeingMetaclassesClearly.html](http://whytheluckystiff.net/articles/seeingMetaclassesClearly.html)
- 

[http://practicalruby.blogspot.com/2007/02/ruby-metaprogramming-introduction.html](http://practicalruby.blogspot.com/2007/02/ruby-metaprogramming-introduction.html)

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 12:34pm UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/6 "2007-11-16T12:34:09Z")

</div>

Hi –

On Fri, 16 Nov 2007, Robert D. wrote:

> Interesting that the Pickaxe got this wrong, never publish code you  
> have not run ;), or maybe this changed between 1.6 and 1.8?

It changed between 1.8.2 and 1.8.6 – see my last response in this  
thread.

David

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 1:00pm UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/7 "2007-11-16T13:00:32Z")

</div>

On Nov 16, 12:32 pm, “David A. Black” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> It will be 🙂 It worked in 1.8.2, doesn’t work in 1.8.6, works again  
> in 1.9. I don’t know what the rationale was for changing the model,  
> but it’s changed back.

AFAIK, there is no rationale for it - it was a bug:  
[http://rubyforge.org/tracker/?func=detail&atid=1698&aid=9462&group\_id=426](http://rubyforge.org/tracker/?func=detail&atid=1698&aid=9462&group_id=426)

Paolo Perrotta  
Bologna, Italy

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 12:32pm UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/8 "2007-11-16T12:32:58Z")

</div>

Hi –

On Fri, 16 Nov 2007, Greg W. wrote:

> PS: I just realized that my (erroneous) expectation
> 
> $Meta\_A.superclass == $Meta\_B  
> $Meta\_B.superclass == $Meta\_Object
> 
> is asserted in Figure 24.2 of the Pickaxe book (2nd edition). Eg, for  
> the second equality, just replace “Guitar” with “B”. So why, I wonder,  
> isn’t it true?

It will be 🙂 It worked in 1.8.2, doesn’t work in 1.8.6, works again  
in 1.9. I don’t know what the rationale was for changing the model,  
but it’s changed back.

rubypal:~$ cat super.rb  
class Object  
def singleton\_class  
class \<\< self; self; end  
end  
end  
p String.singleton\_class.superclass == Object.singleton\_class

rubypal:~$ /usr/local/lib/ruby-1.8.2/bin/ruby -v super.rb  
ruby 1.8.2 (2004-12-25) [i686-linux]  
true  
rubypal:~$ ruby -v super.rb  
ruby 1.8.6 (2007-03-13 patchlevel 0) [i686-linux]  
false  
rubypal:~$ /usr/local/lib/ruby-svn/bin/ruby -v super.rb  
ruby 1.9.0 (2007-11-07 patchlevel 0) [i686-linux]  
true

David

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 8:31pm UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/9 "2007-11-16T20:31:19Z")

</div>

_Many_ thanks for the info. I was going to swallow the 1.8.6 behavior,  
but I much prefer the Figure 24.2 behavior.

On the other hand, it is not true that A.class == $Meta\_A. For the  
“class” message, the rule seems to be that you follow the inheritance  
chain until you find a non-virtual class. This would be a reasonable  
behavior for the “superclass” message too, I think. (Preferable,  
perhaps.)

As for “Meta”: I don’t know if the English language is precise on this  
point. As I understand it, a meta-foo is a foo _about_ foos. A  
meta-class is a class with class instances, I would think. Although  
most Ruby meta-classes are virtual, we can still mix modules into them,  
so they provide a powerful mechanism for meta-programming. That’s why  
I’m inclined to call them (limited) meta-classes.

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 4:57pm UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/10 "2007-11-16T16:57:57Z")

</div>

On Nov 16, 2007 7:00 AM, Paolo Nusco P.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> On Nov 16, 12:32 pm, “David A. Black” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:
> 
> > It will be 🙂 It worked in 1.8.2, doesn’t work in 1.8.6, works again  
> > in 1.9. I don’t know what the rationale was for changing the model,  
> > but it’s changed back.
> 
> AFAIK, there is no rationale for it - it was a bug:  
> [http://rubyforge.org/tracker/?func=detail&atid=1698&aid=9462&group\_id=426](http://rubyforge.org/tracker/?func=detail&atid=1698&aid=9462&group_id=426)

I notice that that bug is still marked as open.

–  
Rick DeNatale

My blog on Ruby  
[http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/)

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 8:52pm UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/11 "2007-11-16T20:52:02Z")

</div>

Hi –

On Sat, 17 Nov 2007, Greg W. wrote:

> point. As I understand it, a meta-foo is a foo _about_ foos. A  
> meta-class is a class with class instances, I would think. Although  
> most Ruby meta-classes are virtual, we can still mix modules into them,  
> so they provide a powerful mechanism for meta-programming. That’s why  
> I’m inclined to call them (limited) meta-classes.

I believe Jim W. once said that the only real metaclass in Ruby  
is Class, since it’s the source of class instances.

David

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 10:02pm UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/12 "2007-11-16T22:02:05Z")

</div>

> I believe Jim W. once said that the only real metaclass in Ruby  
> is Class, since it’s the source of class instances.

True if “real” means “non-virtual”, as it so often does. But that  
leaves  
virtual metaclasses, at the heart of Ruby metaprogramming.

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 10:49pm UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/13 "2007-11-16T22:49:44Z")

</div>

Hi –

On Sat, 17 Nov 2007, Greg W. wrote:

> > I believe Jim W. once said that the only real metaclass in Ruby  
> > is Class, since it’s the source of class instances.
> 
> True if “real” means “non-virtual”, as it so often does. But that  
> leaves  
> virtual metaclasses, at the heart of Ruby metaprogramming.

I’m not sure in what sense they’re virtual (though I know that  
terminology occurs in some error messages). Once they’re created, they  
exist as first-class objects.

David

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 10:52pm UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/14 "2007-11-16T22:52:40Z")

</div>

On Nov 16, 2007 2:51 PM, David A. Black [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Hi –
> 
> On Sat, 17 Nov 2007, Greg W. wrote:

> > As for “Meta”: I don’t know if the English language is precise on this  
> > point. As I understand it, a meta-foo is a foo _about_ foos. A  
> > meta-class is a class with class instances, I would think. Although  
> > most Ruby meta-classes are virtual, we can still mix modules into them,  
> > so they provide a powerful mechanism for meta-programming. That’s why  
> > I’m inclined to call them (limited) meta-classes.
> 
> I believe Jim W. once said that the only real metaclass in Ruby  
> is Class, since it’s the source of class instances.

In Smalltalk, where this stuff isn’t so hidden. a metaclass is a class  
of a class.

The main layout of classes and singleton classes as a parallel  
hierarchy with a cross-over and a loop at the top in Ruby is  
remarkably similar to that of Smalltalk. Of course Smalltalk also  
doesn’t interpose module proxies as class-like things marked to be  
special, since it doesn’t have modules.

On the other hand Ruby doesn’t have a Metaclass class like Smalltalk  
does. Smalltalk metaclasses have behavior mostly for interacting with  
the IDE. Sometimes I wish that Matz had put in a Metaclass class just  
so things wouldn’t be so damned mysterious.

IIRC the quote from Matz in the Pickaxe was that these things we are  
talking about “act just like” Smalltalk Metaclasses, but they aren’t  
metaclasses, they are singleton classes of classes. I don’t know if  
it’s how Matz intended it, but I can interpret it as meaning that they  
aren’t metaclasses just because there isn’t a Metaclass class. In any  
event, I haven’t found much interference from knowing how Smalltalk  
classes and metaclasses interact in understanding Ruby.

–  
Rick DeNatale

My blog on Ruby  
[http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/)

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 11:02pm UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/15 "2007-11-16T23:02:36Z")

</div>

On Sat, Nov 17, 2007 at 04:31:16AM +0900, Greg W. wrote:

> As for “Meta”: I don’t know if the English language is precise on this  
> point. As I understand it, a meta-foo is a foo _about_ foos. A  
> meta-class is a class with class instances, I would think. Although  
> most Ruby meta-classes are virtual, we can still mix modules into them,  
> so they provide a powerful mechanism for meta-programming. That’s why  
> I’m inclined to call them (limited) meta-classes.

Totally, I always think of meta- as meaning “once removed” in a  
parallel fashion more than a compound fashion, you know? Rather than  
strictly saying a “class about classes,” I think it’s also kind of  
a magnetic lasso around “classes informed about classes” or “an  
annex to a class.”

I guess metadata is straightforwardly “data about data,” but data is  
a simple term. Class has complexities (definitions as well as  
storage space) which give ambiguity to the meta- part I think.

Anyway, if you look at the use of the term on the web, like in  
naming sites, such as metafilter or metacritic: I don’t think it is  
implied that these sites would be “filters about filters” or  
“critics criticizing other critics,” but simply that they are once  
removed, culling information from other sites of a similar nature,  
operating in parallel rather than compoundly. I don’t know, I’m  
just trying to give some other evidence of how elastic that little  
prefix can be.

Maybe such things would be better suited with a super- but I like  
both English and Ruby for the ways you have a bit of… errr…  
poetic license, I guess.

\_why

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 16, 2007, 11:27pm UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/16 "2007-11-16T23:27:46Z")

</div>

> > True if “real” means “non-virtual”, as it so often does. But that  
> > leaves  
> > virtual metaclasses, at the heart of Ruby metaprogramming.
> 
> I’m not sure in what sense they’re virtual (though I know that  
> terminology occurs in some error messages). Once they’re created, they  
> exist as first-class objects.

Right. But they can’t be instantiated, and they are specially handled  
by various reflection methods (eg, #class completely ignores them). The  
term “virtual” didn’t appeal to me much either, but Pickaxe and the  
error messages both use it.

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 17, 2007, 12:22am UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/17 "2007-11-17T00:22:46Z")

</div>

On Nov 16, 2007 5:27 PM, Greg W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > > True if “real” means “non-virtual”, as it so often does. But that  
> > > leaves  
> > > virtual metaclasses, at the heart of Ruby metaprogramming.
> > 
> > I’m not sure in what sense they’re virtual (though I know that  
> > terminology occurs in some error messages). Once they’re created, they  
> > exist as first-class objects.
> 
> Right. But they can’t be instantiated,

Well, actually they each have one instance, which is created at the  
same time they are (or vice-versa).

This is the sense that they are ‘singletons’.

And I can think of at least three other classes which have single  
instances and can’t be ‘further’ instantiated, although I’ve never  
heard NilClass, TrueClass, and FalseClass referred to as singleton  
classes.

> and they are specially handled  
> by various reflection methods (eg, #class completely ignores them).

And superclass seems unsure about them from release to release.

> The  
> term “virtual” didn’t appeal to me much either, but Pickaxe and the  
> error messages both use it.

Right, as far as I can see it only comes up in the error messages when  
you try to subclass a class which has the FL\_SINGLETON flag bit set.

–  
Rick DeNatale

My blog on Ruby  
[http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/)

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 17, 2007, 12:48am UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/18 "2007-11-17T00:48:29Z")

</div>

On Sat, Nov 17, 2007 at 07:01:08AM +0900, \_why wrote:

> Maybe such things would be better suited with a super- but I like  
> both English and Ruby for the ways you have a bit of… errr…  
> poetic license, I guess.

Meta is a prefix of Greek origin[1]. The Latin would be ``post’', which  
leads me to two thoughts.

- A ruby metaclass really is like a postclass, which is pretty neat.

- We’re overusing prefixes like meta when so many options exist, such as  
postclass, or what about panclass, hypoclass, or epiclass[2]? I know,  
there’s eigenclass, but does that really clarify it for anyone?

Metaclass is already embedded in the culture, but I nonetheless  
find this to be a useful way to reason about the concept and how it fits  
into the language.

[1] [Meta - Wikipedia](http://en.wikipedia.org/wiki/Meta)  
[2] [English prefix - Wikipedia](http://en.wikipedia.org/wiki/English_prefixes)

---

<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 17, 2007, 1:44am UTC](https://www.ruby-forum.com/t/meta-class-subclass-relationships/120371/19 "2007-11-17T01:44:36Z")

</div>

On Nov 16, 2007 6:47 PM, [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> - We’re overusing prefixes like meta when so many options exist, such as  
> postclass, or what about panclass, hypoclass, or epiclass[2]? I know,  
> there’s eigenclass, but does that really clarify it for anyone?
> 
> Metaclass is already embedded in the culture, but I nonetheless  
> find this to be a useful way to reason about the concept and how it fits  
> into the language.
> 
> [1] [Meta - Wikipedia](http://en.wikipedia.org/wiki/Meta)

# Excerpting from that article:

In epistemology, the prefix meta- is used to mean about (its own  
category). For example, metadata are data about data (who has produced  
them, when, what format the data are in and so on). Similarly,  
metamemory in psychology means an individual’s knowledge about whether  
or not they would remember something if they concentrated on recalling  
it. Furthermore, metaemotion in psychology means an individual’s  
emotion about his/her own basic emotion, or somebody else’s basic  
emotion

…

Etymology

The prefix is derived by back-formation from the Greek preposition and  
prefix meta- (μετά) which meant either “after”, “beside” or “with”.  
Meta- & Meso- are thought to have come into Greek together from a  
mutual cognate, which would further imply ‘meta’ to contain or be of  
the meaning “parallel”. [1]

Quine and Hofstadter

The OED cites uses of the meta- prefix as “beyond, about” (such as  
meta-economics and meta-philosophy) going back to 1917. However, these  
formations are directly parallel to the original “metaphysics” and  
“metaphysical”, that is, as a prefix to general nouns (fields of  
study) or adjectives. Going by the OED citations, it began to be used  
with specific nouns in connection with mathematical logic sometime  
before 1929. (In 1920 David Hilbert proposed a research project in  
what was called “metamathematics.”)

A notable early citation is Quine’s 1937 use of the word  
“metatheorem”, where meta- clearly has the modern meaning of “an X  
about X”. (Note that earlier uses of “meta-economics” and even  
“metaphysics” do not have this doubled conceptual structure, they are  
about or beyond X but they do not themselves constitute an X). Note  
also that this modern meaning allows for self-reference, since if  
something is about the category to which it belongs, it can be about  
itself; it is therefore no coincidence that we find Quine, a  
mathemetician interested in self-reference, using it.

# Douglas Hofstadter, in his 1979 book Gödel, Escher, Bach (and in the sequel, Metamagical Themas), popularized this meaning of the term. This book, which deals extensively with self-reference and touches on Quine and his work, was influential in many computer-related subcultures, and is probably largely responsible for the popularity of the prefix, for its use as a solo term, and for the many recent coinages which use it. Hofstadter uses the meta as a stand-alone word, both as an adjective and as a directional preposition (“going meta”, a term he coins for the old rhetorical trick of taking a debate or analysis to another level of abstraction, as in “This debate isn’t going anywhere.”). This book is also probably responsible for the direct association of “meta” with self-reference, as opposed to just abstraction. The sentence “This sentence contains thirty six letters.” along with the sentence it is embedded in are examples of sentences that reference themselves in this way.

This is the sense of meta in metaclass. First it was adopted from the  
greek meta. Although the Greek meta and Latin post might have some  
meanings in common. I’ve got a hard time understanding how postclass  
would mean anything like metaclass. and postmemory for metamemory,  
postemotion for metaemotion and postdata for metadata just don’t ring  
true.

The reference to Hoftstadter’s GEB and it’s publication date of 1979  
is interesting. I can’t remember exactly when in the evolution of  
Smalltalk the term metaclass was born, but it was definitely there in  
the 1980 version.

It just so happens that I pulled my old copy of GEB down from the  
attic a week or so ago. Rereading it has been competing for my  
reading time with “SQL for MySQL Developers.” An interesting note is  
that back then a 777 page textbook sold for $17.50 back then!

–  
Rick DeNatale

My blog on Ruby  
[http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/)