Editing Metaclass (section)

==In Ruby==
Ruby purifies the Smalltalk-80 concept of metaclasses by introducing [[wiktionary:eigenclasses|eigenclasses]],
removing the <code>Metaclass</code> class,
and (un)redefining the class-of map.
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The change can be schematized as follows:<ref>{{cite web | url=http://www.atalon.cz/rb-om/ruby-object-model/co-smalltalk/ | title=The Ruby Object Model: Comparison with Smalltalk-80 }}</ref>
{| border=0 cellpadding=0 style="margin: 2ex auto"
|-
|
{| class="wikitable"
|+ Smalltalk-80
|-
| <div style="height: 5ex; margin:1ex; padding: 1ex; border: 1px dotted gray;"><span style="position:relative; top:1ex;">Classes</span></div>
| <div style="height: 5ex; margin:1ex; padding: 1ex 0.4ex; border: 1px dotted gray;"><span style="font-size:85%;">'''Implicit<br>metaclasses'''</span></div>
| rowspan="2" style="width:4ex;" | &nbsp;&nbsp;
|-
| <div style="height: 5ex; margin:1ex; padding: 1ex; border: 1px dotted gray;">Terminal<br>objects</div>
|
|}
| style="font-size: 200%; vertical-align: middle; padding: 1px 20px;" | →
|
{| class="wikitable"
|+ Ruby
|-
| <div style="height: 5ex; margin:1ex; padding: 1ex; border: 1px dotted gray;"><span style="position:relative; top:1ex;">Classes</span></div>
| <div style="height: 5ex; margin:1ex; padding: 1ex 0.4ex; border: 1px dotted gray;"><span style="font-size:85%;">'''Eigenclasses of<br>classes'''</span></div>
| rowspan=2 | <div style="height: 12ex; margin:1ex; padding: 2ex 0.4ex; border: 1px dotted gray;"><span style="font-size:85%;">'''Eigenclasses<br>of<br>eigenclasses'''</span></div>
|-
| <div style="height: 5ex; margin:1ex; padding: 1ex; border: 1px dotted gray;">Terminal<br>objects</div>
| <div style="height: 5ex; margin:1ex; padding: 1ex 0.4ex; border: 1px dotted gray;"><span style="font-size:85%;">'''Eigenclasses of<br>terminal objects'''</span></div>
|}
|}
Note in particular the correspondence between Smalltalk's implicit metaclasses and Ruby's eigenclasses of classes.
The Ruby eigenclass model makes the concept of implicit metaclasses fully uniform: every object ''x'' has its own meta-object, called the ''eigenclass'' of ''x'', which is one meta-level higher than ''x''. The "higher order" eigenclasses usually exist purely conceptually – they do not contain any methods or store any (other) data in most Ruby programs.<ref>
{{cite book | publisher=Pragmatic Bookshelf | author=Paolo Perrotta | url=http://pragprog.com/book/ppmetr/metaprogramming-ruby | title=Metaprogramming Ruby | year=2010 | isbn=978-1-934356-47-0}}</ref>

The following diagrams show a sample core structure of Smalltalk-80 and Ruby in comparison.<ref>{{cite web | url=http://www.atalon.cz/om/object-membership/ | title=Object Membership: The Core Structure of Object Technology }}</ref>
In both languages, the structure consists of a built-in part which contains the circular objects (i.e. objects that appear in a cycle formed by a combination of blue or green links) and a user-part which has four explicit objects: classes <code>A</code> and <code>B</code>
and terminal objects <code>u</code> and <code>v</code>.
Green links show the child→parent relation of inheritance (with the implicit upward direction), blue links show the complementary member→container relation of instantiation (a blue link from ''x'' points to the least actual container of ''x'' that is the start point for the method lookup when a method is invoked on ''x''). Gray nodes display the eigenclasses (resp. implicit metaclasses in the case of Smalltalk-80).
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 (<code>A</code> is a direct subclass of <code>Object</code>, <code>B</code> is a direct subclass of <code>A</code> and both <code>u</code> and <code>v</code> are direct instances of <code>B</code>).
-->

{| border=0 cellpadding=0 cellspacing=0 style="margin: 2ex auto"
|-
| style="text-align:center" | Smalltalk-80|| style="width:4ex" |&nbsp;|| style="text-align:center" |Ruby
|-
| [[File:Smalltalk-metaclass-sample.svg|300px|Implicit metaclasses in Smalltalk-80 - A sample structure]]||
||[[File:Ruby-metaclass-sample.svg|500px|Eigenclasses in Ruby - A sample structure]]
|}

The diagram on the right also provides a picture of [[lazy evaluation]] of eigenclasses in Ruby. The <code>v</code> object can have its eigenclass evaluated (allocated) as a consequence of adding ''singleton methods'' to <code>v</code>.

According to the Ruby's introspection method named <code>class</code>,
the class of every class (and of every eigenclass) is
constantly the <code>Class</code> class (denoted by <code>c</code> in the diagram).
<code>Class</code>, and <code>Struct</code> are the only classes that have classes as instances.<ref>{{cite web|title=Struct|url=https://docs.ruby-lang.org/en/master/Struct.html}}</ref> {{Disputed inline|The_Struct_class_in_Ruby|date=May 2015}} Subclassing of <code>Class</code> is disallowed.
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Following the standard definition of metaclasses we can conclude that <code>Class</code> and <code>Struct</code> are the only metaclasses in Ruby.
This seems to contradict the correspondence between Ruby and Smalltalk,
since in Smalltalk-80, every class has its own metaclass.
The discrepancy is based on the disagreement between
the <code>class</code> introspection method in Ruby and Smalltalk. While the map ''x ↦ x.''<code>class</code> coincides on terminal objects, it differs in the restriction to classes. As already mentioned above, for a class <code>''x''</code>, the Ruby expression <code>''x''.class</code> evaluates constantly to <code>Class</code>. In Smalltalk-80, if <code>''x''</code> is a class then the expression <code>''x'' class</code> corresponds
to the Ruby's <code>''x''.singleton_class</code>
– which evaluates to the eigenclass of <code>''x''</code>.