# Defining Eigenclass **URL:** \ **Category:** Ruby\ **Created:** [August 15, 2006, 2:16pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358 "2006-08-15T14:16:02Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:16pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/1 "2006-08-15T14:16:02Z")
So, what is an eigenclass? I googled it, and couldn’t find anything about it (other than Mauricio’s great blog/wiki). Wikipedia doesn’t have anything on it other than an indirect relation to Metaprogramming (which I already know it’s a part of, just not sure wherein). Can someone provide some information on the idea of an eigenclass? Thanks, M.T. ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:16pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/2 "2006-08-15T14:16:02Z")
Google Singleton class. That’ll surely help 😉 ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:16pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/3 "2006-08-15T14:16:06Z")
Matt T. wrote: > So, what is an eigenclass? I googled it, and couldn’t find anything > about it (other than Mauricio’s great blog/wiki). Wikipedia doesn’t > have anything on it other than an indirect relation to Metaprogramming > (which I already know it’s a part of, just not sure wherein). > > Can someone provide some information on the idea of an eigenclass? I’ll try. (I call it “singleton class” since that is what I learned.) It’s the “place” where things go that are associated with a specific _object_ rather than a _class_ of objects. For example, if I add a method to an object, that method “lives” in the singleton class. superman = “Clark Kent” def superman.fly puts “Look! I’m flying!” end Here the “fly” method is certainly not part of the String class. No other string responds to that method – only this one. It’s a singleton method, and it “lives” in the “singleton class.” You can see more clearly the class-like nature of this thing if we use this syntax: class \<\< superman def fly #… end end That at least “looks” like a class. But also be aware: 1. It’s not a “real” class in that it can’t be instantiated. 2. Matz has said that this “thing” _may_ not be considered a class in the future (IIRC). Note that we can capture the actual value of the singleton class if we want to: ssc = class \<\< superman # ssc = superman’s singleton class self # return self as last expression end puts ssc.class # Class Finally, note that what we call “class methods” are just a special case of singleton methods. Suppose Dog inherits from Mammal; then suppose we make a Dog.bark class method. Where does “bark” live? Clearly not “in” Dog (as an instance method would be “in” it). Not in the parent class, either. It lives in the singleton class of Dog. So a “class method” is really just “a singleton method on an object which happens to be a class.” Does that help any? Hal ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:16pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/4 "2006-08-15T14:16:51Z")
On 8/11/06, Trans [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Google Singleton class. That’ll surely help 😉 The ‘official’ ruby docs talk about singleton classes, but eigenclass seems to have been proposed by someone, somewhere, sometime as an alternative. This seems to be confined to the ruby community and provides the subtext for things like [eigenclass.org](http://eigenclass.org), but I can’t seem to find where the term came from originally. I’d love enlightenment on that. Now I guess that the etymology comes from the German word “eigen” which can be translated to “own” or “individual”, and also forms the root of the mathematical terms “eigenvector” and “eigenvalue”. If we look at “eigenclass” from the perspective of “eigenvector” though, it doesn’t fit that well, since an “eigenvector” of a transformation in linear algebra is a vector which is unchanged in direction (or more properly only changes by being multiplied by a scalar) under that transformation. – Rick DeNatale [http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/) ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:16pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/5 "2006-08-15T14:16:38Z")
Yeah, that was very clear. Thanks for the explanation. I just didn’t connect Eigenclasses with Singleton classes. But, actually, that’s a good description of Singleton classes as well. Thanks! M.T. ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:16pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/6 "2006-08-15T14:16:55Z")
Rick DeNatale wrote: > The ‘official’ ruby docs talk about singleton classes, but eigenclass > seems to have been proposed by someone, somewhere, sometime as an > alternative. I recalled seeing the discussion in which eigenclass was first proposed. Here’s the post by csaba on April 21, 2005: "Own class" is the best I've heard 'till now in terms of correctness, it's just a bit "pale". I mean, when you say "I go there by my own car", then the "own" doesn't refer to a special type of car, it just refers to some relation of the car with other things. It refers to a non-intrinsic thing. But if you say "I go there by my batcar", that makes a difference. Such a thing is what we need for ruby. What about "eigenclass", like in eigenvalue? Which was part of the “Article: Seeing Metaclasses Clearly” thread started by \_why on April 17, 2005. See [http://tinyurl.com/mjy3m](http://tinyurl.com/mjy3m). I much prefer the name “eigenclass” to “singleton class” when referring to an object’s own class. Avoids people having to always qualify what they mean by singleton class, as you will constantly see people say things like “when you have a Singleton class (one that include Singleton, not the eigenclass)”, which is just annoying. The term singleton has already been taken by a design pattern. Besides, eigenclass is just a neat word. =) Tom ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:16pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/7 "2006-08-15T14:16:59Z")
ts wrote: > Indeed the source code tells us what it calls the thing (singleton class), but should that forbid us from adopting a different, less ambiguous term when writing about and discussing the concept? Tom ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:16pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/8 "2006-08-15T14:16:55Z")
> > > > > “T” == Tom W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: T\> to an object’s own class. Avoids people having to always qualify what T\> they mean by singleton class, as you will constantly see people say Never look at the ruby source, or you’ll have a surprise moulon% grep ^rb\_singleton\_class ruby-1.8.5/\*.c ruby-1.8.5/class.c:rb\_singleton\_class\_clone(obj) ruby-1.8.5/class.c:rb\_singleton\_class\_attached(klass, obj) ruby-1.8.5/class.c:rb\_singleton\_class(obj) moulon% Guy Decoux ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:16pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/9 "2006-08-15T14:16:59Z")
> > > > > “T” == Tom W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: T\> Indeed the source code tells us what it calls the thing (singleton T\> class), but should that forbid us from adopting a different, less T\> ambiguous term when writing about and discussing the concept? It’s ambiguous only for you 🙂 What is the next step, eigen method ? Guy Decoux ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:17pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/10 "2006-08-15T14:17:00Z")
ts wrote: > T\> Indeed the source code tells us what it calls the thing (singleton > T\> class), but should that forbid us from adopting a different, less > T\> ambiguous term when writing about and discussing the concept? > > It’s ambiguous only for you 🙂 If that were true, we wouldn’t be having this discussion. =) Also, see the “nonce” thread going right now for others struggling with the less-than-stellarness of the currently used “singleton class”. Tom ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:17pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/11 "2006-08-15T14:17:01Z")
> > > > > “T” == Tom W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: T\> Also, see the “nonce” thread going right now for others struggling with T\> the less-than-stellarness of the currently used “singleton class”. This is the same thread, made by someone which has always had problems with this term … Rien de nouveau sous le soleil … Guy Decoux ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:17pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/12 "2006-08-15T14:17:10Z")
On Sat, Aug 12, 2006 at 02:19:29AM +0900, ts wrote: > > > > > > “T” == Tom W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: > > T\> Also, see the “nonce” thread going right now for others struggling with > T\> the less-than-stellarness of the currently used “singleton class”. > > This is the same thread, made by someone which has always had problems > with this term … Funny . . . I’ve seen several people suggesting other names for it, including David Black’s “own class” recommendation (which predates “eigenclass” as a suggestion). ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:17pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/13 "2006-08-15T14:17:15Z")
Hi – On Sat, 12 Aug 2006, Chad P. wrote: > Funny . . . I’ve seen several people suggesting other names for it, > including David Black’s “own class” recommendation (which predates > “eigenclass” as a suggestion). The history of it, as I recall it, is that Matz doesn’t want to implement RCR 231 ([http://www.rcrchive.net/rcr/show/231](http://www.rcrchive.net/rcr/show/231)), unless and until he comes up with what he considers the perfect name. Then there’s the “What does Matz have to do with it?”, fait-accompli campaign for eigenclass 🙂 And various other branchings off, generalizations of “metaclass”, and so on – so that, unfortunately, the singleton class is now most commonly known as something like: this thing that doesn’t really have a name and is known sometimes as singleton class, but also as eigenclass, or metaclass, or something-else-class – and it’s really, really confusing and hard to understand… David ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:17pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/14 "2006-08-15T14:17:15Z")
On Sat, Aug 12, 2006 at 03:44:40AM +0900, removed\_email\_address@domain.invalid wrote: > singleton class, but also as eigenclass, or metaclass, or > something-else-class – and it’s really, really confusing and > hard to understand… Works for me. Let’s call it that. ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:17pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/15 "2006-08-15T14:17:32Z")
Chad P. wrote: > > this thing that doesn’t really have a name and is known sometimes as > > singleton class, but also as eigenclass, or metaclass, or > > something-else-class – and it’s really, really confusing and > > hard to understand… > > Works for me. Let’s call it that. Great! But put underscores in there for the RCR. Hal ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:17pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/16 "2006-08-15T14:17:33Z")
On Sat, Aug 12, 2006 at 08:19:28AM +0900, Hal F. wrote: > > > this thing that doesn’t really have a name and is known sometimes as > > > singleton class, but also as eigenclass, or metaclass, or > > > something-else-class – and it’s really, really confusing and > > > hard to understand… > > > > Works for me. Let’s call it that. > > Great! But put underscores in there for the RCR. Oh, of course. I’m inspired to make an industrial cover of the Depeche Mode song “Personal Jesus”. I’ll call it Eigensavior. ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:17pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/17 "2006-08-15T14:17:44Z")
Rick DeNatale wrote: > Hmpf … I just realized that the typical definition of an eigenvector is A vector ‘x’ such that ‘A_x = lambda_x’, where A is a square matrix. ‘lambda’ is the eigenvalue. So, was eigenclass named after the eigenvalue or the eigenvector? ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:18pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/18 "2006-08-15T14:18:05Z")
Hal F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: > > > this thing that doesn’t really have a name and is known sometimes as > > > singleton class, but also as eigenclass, or metaclass, or > > > something-else-class – and it’s really, really confusing and > > > hard to understand… > > > Works for me. Let’s call it that. > > Great! But put underscores in there for the RCR. We could add some magic that every method matching /\_class/ returns the singleton class… 🙂 foo.eigen\_class foo.singleton\_class foo.ueber\_class ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:18pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/19 "2006-08-15T14:18:15Z")
> We could add some magic that every method matching /\_class/ returns > the singleton class… 🙂 > > foo.eigen\_class > foo.singleton\_class > foo.ueber\_class Hehe. That’s fitting. Well, we could drop all pretenses foo.\_class And call it the “under class”. T. ---
**Author:** ![maraby](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/f08c70/32.png) [@maraby](https://www.ruby-forum.com/u/maraby)\ **Post date:** [August 15, 2006, 2:18pm UTC](https://www.ruby-forum.com/t/defining-eigenclass/102358/20 "2006-08-15T14:18:41Z")
Ummmm… class object? Like, not the instance of the class, but the class object itself. Would that be too transparent? 🙂 (Or, alternatively… just plain wrong… if I’m being slow.) M.T. [Next page](https://www.ruby-forum.com/t/defining-eigenclass/102358.md?page=2)