Singleton Modules rather than Singleton Classes

Discussion in 'Ruby' started by Trans, Sep 1, 2007.

  1. Trans

    Trans Guest

    This recently came up in the thread entitled "Python-style
    Decorators", so I thought it a fair idea to "formally" put it before
    the Ruby community. Here's the deal...

    Singleton classes already have more in common with modules than
    classes by the very behaviors that distinguish a module from a class --
    they cannot be instantiated and they can not be inherited. So why
    exactly do we deem them classes at all? If instead we took them to be
    actual modules, and used as such, it would open up some really nice
    possibilities. For example:

    class A
    def self.x
    "x"
    end
    end

    class B
    extend (class << A; self; end)
    end

    B.x #=> "x"

    This makes it dead simple to pass along module class-level methods in
    the class hierarchy. The immediate use of this change is apparent --no
    more ClassMethods/included callback hacks required.

    One thing to note about this example, the notation "(class << A; self;
    end)" becomes a bit of a misnomer in light of the suggested change.
    "A.singleton" or some other method name, would be far better.

    Another possibility, which derives from the aforementioned thread, are
    method decorators using singleton def notation:

    class Example

    def memoized.foo
    ...
    end

    end

    It is not possible to meta-code this currently b/c the definition of
    foo gets locked away in a singleton class where it can not be reused.
    If that singleton could be included into a class (in this case
    Example) then presto, problem solved, and a powerful new notation is
    opened up to the Ruby programmer.

    T.
     
    Trans, Sep 1, 2007
    #1
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  2. On 9/1/07, Trans <> wrote:
    > This recently came up in the thread entitled "Python-style
    > Decorators", so I thought it a fair idea to "formally" put it before
    > the Ruby community. Here's the deal...
    >
    > Singleton classes already have more in common with modules than
    > classes by the very behaviors that distinguish a module from a class --
    > they cannot be instantiated and they can not be inherited. So why
    > exactly do we deem them classes at all? If instead we took them to be
    > actual modules, and used as such, it would open up some really nice
    > possibilities. For example:
    >


    Counter proposal: remove singleton classes all together in favor of
    simply having singleton methods. The useful facility of a singleton
    class is that it allows you to have per-object methods (singleton
    methods). Class and module are both misnomers for the "thing that acts
    as a place to store singleton methods." By eliminating the visibility
    of this implementation detail, it allows for other implementations to
    be experimented with. The more you pin down how singleton methods are
    implemented, and the more you start doing things with details of that
    implementation the hard it becomes to have flexibility in that
    implementation. This also removes the need to justify referring to
    them as classes, and the false expectations this creates. My 2cents..
    >
    >
     
    Logan Capaldo, Sep 1, 2007
    #2
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  3. Trans

    Guest

    Hi --

    On Sun, 2 Sep 2007, Logan Capaldo wrote:

    > On 9/1/07, Trans <> wrote:
    >> This recently came up in the thread entitled "Python-style
    >> Decorators", so I thought it a fair idea to "formally" put it before
    >> the Ruby community. Here's the deal...
    >>
    >> Singleton classes already have more in common with modules than
    >> classes by the very behaviors that distinguish a module from a class --
    >> they cannot be instantiated and they can not be inherited. So why
    >> exactly do we deem them classes at all? If instead we took them to be
    >> actual modules, and used as such, it would open up some really nice
    >> possibilities. For example:
    >>

    >
    > Counter proposal: remove singleton classes all together in favor of
    > simply having singleton methods. The useful facility of a singleton
    > class is that it allows you to have per-object methods (singleton
    > methods). Class and module are both misnomers for the "thing that acts
    > as a place to store singleton methods." By eliminating the visibility
    > of this implementation detail, it allows for other implementations to
    > be experimented with. The more you pin down how singleton methods are
    > implemented, and the more you start doing things with details of that
    > implementation the hard it becomes to have flexibility in that
    > implementation. This also removes the need to justify referring to
    > them as classes, and the false expectations this creates. My 2cents..


    But then you introduce a whole second model of how method lookup and
    so forth works. What's nice about singleton classes is that they fit
    into the same basic model as other classes; once the premise is
    granted that every object can have a singleton class as well as a
    "birth" class, it all flows from there. I like the fact that every
    method lives in a class or module.

    I would actually be happy for them to be singleton modules instead of
    classes, though I don't really like the idea that one object can
    extend itself with another object's singleton methods. Or, to put it
    another way, I do like the possibility of strictly per-object
    behavior, so I wouldn't want to see that done away with. Maybe if it
    were done explicitly via dup'ing of some kind it would be OK.
    Otherwise it's just multiple objects sharing a module, which is
    basically what the non-singleton scenario already is.


    David

    --
    * Books:
    RAILS ROUTING (new! http://www.awprofessional.com/title/0321509242)
    RUBY FOR RAILS (http://www.manning.com/black)
    * Ruby/Rails training
    & consulting: Ruby Power and Light, LLC (http://www.rubypal.com)
     
    , Sep 1, 2007
    #3
  4. Trans

    Robert Dober Guest

    On 9/1/07, <> wrote:
    > Hi --
    >
    > On Sun, 2 Sep 2007, Logan Capaldo wrote:
    >
    > > On 9/1/07, Trans <> wrote:
    > >> This recently came up in the thread entitled "Python-style
    > >> Decorators", so I thought it a fair idea to "formally" put it before
    > >> the Ruby community. Here's the deal...
    > >>
    > >> Singleton classes already have more in common with modules than
    > >> classes by the very behaviors that distinguish a module from a class --
    > >> they cannot be instantiated and they can not be inherited. So why
    > >> exactly do we deem them classes at all? If instead we took them to be
    > >> actual modules, and used as such, it would open up some really nice
    > >> possibilities. For example:
    > >>

    > >
    > > Counter proposal: remove singleton classes all together in favor of
    > > simply having singleton methods. The useful facility of a singleton
    > > class is that it allows you to have per-object methods (singleton
    > > methods). Class and module are both misnomers for the "thing that acts
    > > as a place to store singleton methods." By eliminating the visibility
    > > of this implementation detail, it allows for other implementations to
    > > be experimented with. The more you pin down how singleton methods are
    > > implemented, and the more you start doing things with details of that
    > > implementation the hard it becomes to have flexibility in that
    > > implementation. This also removes the need to justify referring to
    > > them as classes, and the false expectations this creates. My 2cents..

    >
    > But then you introduce a whole second model of how method lookup and
    > so forth works. What's nice about singleton classes is that they fit
    > into the same basic model as other classes; once the premise is
    > granted that every object can have a singleton class as well as a
    > "birth" class, it all flows from there. I like the fact that every
    > method lives in a class or module.
    >
    > I would actually be happy for them to be singleton modules instead of
    > classes, though I don't really like the idea that one object can
    > extend itself with another object's singleton methods. Or, to put it
    > another way, I do like the possibility of strictly per-object
    > behavior, so I wouldn't want to see that done away with. Maybe if it
    > were done explicitly via dup'ing of some kind it would be OK.
    > Otherwise it's just multiple objects sharing a module, which is
    > basically what the non-singleton scenario already is.
    >
    >
    > David
    >
    > --
    > * Books:
    > RAILS ROUTING (new! http://www.awprofessional.com/title/0321509242)
    > RUBY FOR RAILS (http://www.manning.com/black)
    > * Ruby/Rails training
    > & consulting: Ruby Power and Light, LLC (http://www.rubypal.com)
    >

    All three proposals make sense to me, what does not make sense to me
    is the current state of affairs, as long as
    instance_eval{
    def a; @a end
    }
    works on arbitrary objects I will stay confused ;).
    It probably all depends on what kind of OO Style one prefers,
    Personally I would be looking forward to some serious simplifications, like e.g.
    * Any unbound method can be bound to any object.
    * Methods and Blocks could be unified.
    * Method definitions in classes behave as in Modules.
    * define_method defines a method on any object
    * define_instance_method defines an instance method on any object.
    * Any object can have instances. (well and classes just went away)

    Well just some ideas more ;)

    Robert
    >



    --
    I'm an atheist and that's it. I believe there's nothing we can know
    except that we should be kind to each other and do what we can for
    other people.
    -- Katharine Hepburn
     
    Robert Dober, Sep 2, 2007
    #4
  5. Trans

    ara.t.howard Guest

    On Sep 1, 2007, at 2:14 PM, Logan Capaldo wrote:

    >
    > Counter proposal: remove singleton classes all together in favor of
    > simply having singleton methods. The useful facility of a singleton
    > class is that it allows you to have per-object methods (singleton
    > methods). Class and module are both misnomers for the "thing that acts
    > as a place to store singleton methods." By eliminating the visibility
    > of this implementation detail, it allows for other implementations to
    > be experimented with. The more you pin down how singleton methods are
    > implemented, and the more you start doing things with details of that
    > implementation the hard it becomes to have flexibility in that
    > implementation. This also removes the need to justify referring to
    > them as classes, and the false expectations this creates. My 2cents..


    i think i've done as much meta-programming as anyone out there and i
    would not like to see that chance. the reason is that the
    singleton_class is also a nice place to store *state* - not only
    methods - when doing deep ruby magic. otherwise the whole affair of
    transforming singleton classes into modules seems reasonable but
    superfluous to me: it's just so easy to do

    module M; def x() 'x' end; end

    class A; extend M; end

    class B; extend M; end

    B.x #=> 'x'

    that is hardly seems worth all the c coding and inevitable bugs even
    though the idea seems sounds. maybe 2.0?

    2cts.


    a @ http://drawohara.com/
    --
    we can deny everything, except that we have the possibility of being
    better. simply reflect on that.
    h.h. the 14th dalai lama
     
    ara.t.howard, Sep 10, 2007
    #5
  6. On 9/10/07, ara.t.howard <> wrote:
    > > i think i've done as much meta-programming as anyone out there and i

    > would not like to see that chance. the reason is that the
    > singleton_class is also a nice place to store *state* - not only
    > methods - when doing deep ruby magic.


    Care to give an example?
     
    Logan Capaldo, Sep 10, 2007
    #6
  7. Trans

    ara.t.howard Guest

    On Sep 10, 2007, at 4:32 PM, Logan Capaldo wrote:

    > Care to give an example?


    sure.

    file: traits.rb
    ...
    8 class Object
    9 #--{{{
    10 def singleton_method_added(*a, &b)
    11 #--{{{
    12 ret = super rescue nil
    13 obj = self
    14 obj.__trait_singleton_class.__trait_module_eval
    { @__trait_singleton_super = obj }
    15 ret
    16 #--}}}
    17 end
    ...

    this object is used in the search path: traits implements attr like
    methods with inheritance.


    a @ http://drawohara.com/
    --
    we can deny everything, except that we have the possibility of being
    better. simply reflect on that.
    h.h. the 14th dalai lama
     
    ara.t.howard, Sep 11, 2007
    #7
  8. 2007/9/1, Trans <>:
    > This recently came up in the thread entitled "Python-style
    > Decorators", so I thought it a fair idea to "formally" put it before
    > the Ruby community. Here's the deal...
    >
    > Singleton classes already have more in common with modules than
    > classes by the very behaviors that distinguish a module from a class --
    > they cannot be instantiated and they can not be inherited. So why
    > exactly do we deem them classes at all? If instead we took them to be
    > actual modules, and used as such, it would open up some really nice
    > possibilities. For example:
    >
    > class A
    > def self.x
    > "x"
    > end
    > end
    >
    > class B
    > extend (class << A; self; end)
    > end
    >
    > B.x #=> "x"
    >
    > This makes it dead simple to pass along module class-level methods in
    > the class hierarchy. The immediate use of this change is apparent --no
    > more ClassMethods/included callback hacks required.


    You do not need that, because the functionality is there already:

    $ ruby <<XXX
    > class A
    > def self.x
    > "x"
    > end
    > end
    > class B < A; end
    > p B.x
    > XXX

    "x"

    > One thing to note about this example, the notation "(class << A; self;
    > end)" becomes a bit of a misnomer in light of the suggested change.
    > "A.singleton" or some other method name, would be far better.
    >
    > Another possibility, which derives from the aforementioned thread, are
    > method decorators using singleton def notation:
    >
    > class Example
    >
    > def memoized.foo
    > ...
    > end
    >
    > end
    >
    > It is not possible to meta-code this currently b/c the definition of
    > foo gets locked away in a singleton class where it can not be reused.
    > If that singleton could be included into a class (in this case
    > Example) then presto, problem solved, and a powerful new notation is
    > opened up to the Ruby programmer.


    I'd generally prefer to define modules explicitly and extend classes
    with them as Ara suggested. IMHO that's a cleaner way to achieve what
    you want and it also documents things more nicely (especially you can
    look at inheritance etc.). My 0.02EUR

    Kind regards

    robert
     
    Robert Klemme, Sep 11, 2007
    #8
  9. On 9/10/07, ara.t.howard <> wrote:
    >
    > On Sep 10, 2007, at 4:32 PM, Logan Capaldo wrote:
    >
    > > Care to give an example?

    >
    > sure.
    >
    > file: traits.rb
    > ...
    > 8 class Object
    > 9 #--{{{
    > 10 def singleton_method_added(*a, &b)
    > 11 #--{{{
    > 12 ret = super rescue nil
    > 13 obj = self
    > 14 obj.__trait_singleton_class.__trait_module_eval
    > { @__trait_singleton_super = obj }
    > 15 ret
    > 16 #--}}}
    > 17 end
    > ...


    I assume __trait_singleton_class is (class << obj; self; end) and
    __trait_module_module is a wrapper around module_eval?

    You can do this without ivars, just define a __trait_singleton_super
    singleton method that returns obj, Or you can do it with ivars but
    keep the ivar in the object instead of the singleton class.

    >
    > this object is used in the search path: traits implements attr like
    > methods with inheritance.
    >
    >
    > a @ http://drawohara.com/
    > --
    > we can deny everything, except that we have the possibility of being
    > better. simply reflect on that.
    > h.h. the 14th dalai lama
    >
    >
    >
    >
    >
     
    Logan Capaldo, Sep 11, 2007
    #9
  10. Trans

    Trans Guest

    On Sep 11, 1:55 am, "Robert Klemme" <>
    wrote:

    > I'd generally prefer to define modules explicitly and extend classes
    > with them as Ara suggested. IMHO that's a cleaner way to achieve what
    > you want and it also documents things more nicely (especially you can
    > look at inheritance etc.). My 0.02EUR


    That is simply not correct. Fist of all, you do not control all code,
    so you can not make someone else modularized their module's class-
    level methods, so you can later augment them with meta-code. Secondly,
    the primary point has nothing to do with the fact that one can manage
    workarounds, of course there are ways, but b/c of limitations they are
    fragile and inefficient. Lastly, you are missing the point when you
    you say, "prefer to define modules explicitly... that's a cleaner way
    to achieve what you want". That's not the issue. Again, I encourage
    you to have a look as Facets' inheritor.rb lib to get a better
    understanding of this.

    T.
     
    Trans, Sep 12, 2007
    #10
  11. 2007/9/12, Trans <>:
    >
    >
    > On Sep 11, 1:55 am, "Robert Klemme" <>
    > wrote:
    >
    > > I'd generally prefer to define modules explicitly and extend classes
    > > with them as Ara suggested. IMHO that's a cleaner way to achieve what
    > > you want and it also documents things more nicely (especially you can
    > > look at inheritance etc.). My 0.02EUR

    >
    > That is simply not correct. Fist of all, you do not control all code,
    > so you can not make someone else modularized their module's class-
    > level methods, so you can later augment them with meta-code.


    I'm not sure. If they are not modularized I probably would not want
    to reuse them.

    > Secondly,
    > the primary point has nothing to do with the fact that one can manage
    > workarounds, of course there are ways, but b/c of limitations they are
    > fragile and inefficient.


    I don' t think inheriting (as show in my first reply) is fragile or
    inefficient but I may of course get in the way with other inheritance
    you'd want to do.

    > Lastly, you are missing the point when you
    > you say, "prefer to define modules explicitly... that's a cleaner way
    > to achieve what you want". That's not the issue. Again, I encourage
    > you to have a look as Facets' inheritor.rb lib to get a better
    > understanding of this.


    Um, you got me stumped here: in your original post you advertised
    easier reuse of singleton methods by making the singleton class a
    singleton module instead. Where exactly am I missing the point?

    Kind regards

    robert
     
    Robert Klemme, Sep 13, 2007
    #11
  12. Trans

    Trans Guest

    On Sep 13, 1:59 am, "Robert Klemme" <>
    wrote:
    > 2007/9/12, Trans <>:
    >
    >
    >
    > > On Sep 11, 1:55 am, "Robert Klemme" <>
    > > wrote:

    >
    > > > I'd generally prefer to define modules explicitly and extend classes
    > > > with them as Ara suggested. IMHO that's a cleaner way to achieve what
    > > > you want and it also documents things more nicely (especially you can
    > > > look at inheritance etc.). My 0.02EUR

    >
    > > That is simply not correct. Fist of all, you do not control all code,
    > > so you can not make someone else modularized their module's class-
    > > level methods, so you can later augment them with meta-code.

    >
    > I'm not sure. If they are not modularized I probably would not want
    > to reuse them.


    Lots of libs define class singleton methods, none of them are
    modularized.

    > > Secondly,
    > > the primary point has nothing to do with the fact that one can manage
    > > workarounds, of course there are ways, but b/c of limitations they are
    > > fragile and inefficient.

    >
    > I don' t think inheriting (as show in my first reply) is fragile or
    > inefficient but I may of course get in the way with other inheritance
    > you'd want to do.


    Sorry, it's my fault. I should have made A a module in my original
    example to demonstrate better. The inheritance of your example isn't
    the difficulty. It's not class to class inheritance that makes things
    tricky, it's the modules. Which is what leads us to use ClassMethods
    modules and included callbacks. Generally, the more meta-code one must
    use (or just plan code for that matter) the more fragile a solution.
    And really, when we are depending on a callback to achieve a common
    pattern, that's a sure sign that something is amiss.

    > > Lastly, you are missing the point when you
    > > you say, "prefer to define modules explicitly... that's a cleaner way
    > > to achieve what you want". That's not the issue. Again, I encourage
    > > you to have a look as Facets' inheritor.rb lib to get a better
    > > understanding of this.

    >
    > Um, you got me stumped here: in your original post you advertised
    > easier reuse of singleton methods by making the singleton class a
    > singleton module instead. Where exactly am I missing the point?


    Because it's not a matter of preference. There is no choice. The only
    way to currently achieve class-level class->module->class inheritance
    is through secondary modules and meta-coding. And I certainly don't
    see how being required to create an artificial abstraction (for
    example, ClassMethods) leads to cleaner code.

    Besides, no amount of meta-coding and secondary modules will allow for
    "fluent singleton notation" at all. Eg.

    def memoized.foo
    ...
    end

    T.
     
    Trans, Sep 13, 2007
    #12
  13. 2007/9/13, Trans <>:
    >
    >
    > On Sep 13, 1:59 am, "Robert Klemme" <>
    > wrote:
    > > 2007/9/12, Trans <>:
    > >
    > >
    > >
    > > > On Sep 11, 1:55 am, "Robert Klemme" <>
    > > > wrote:

    > >
    > > > > I'd generally prefer to define modules explicitly and extend classes
    > > > > with them as Ara suggested. IMHO that's a cleaner way to achieve what
    > > > > you want and it also documents things more nicely (especially you can
    > > > > look at inheritance etc.). My 0.02EUR

    > >
    > > > That is simply not correct. Fist of all, you do not control all code,
    > > > so you can not make someone else modularized their module's class-
    > > > level methods, so you can later augment them with meta-code.

    > >
    > > I'm not sure. If they are not modularized I probably would not want
    > > to reuse them.

    >
    > Lots of libs define class singleton methods, none of them are
    > modularized.
    >
    > > > Secondly,
    > > > the primary point has nothing to do with the fact that one can manage
    > > > workarounds, of course there are ways, but b/c of limitations they are
    > > > fragile and inefficient.

    > >
    > > I don' t think inheriting (as show in my first reply) is fragile or
    > > inefficient but I may of course get in the way with other inheritance
    > > you'd want to do.

    >
    > Sorry, it's my fault. I should have made A a module in my original
    > example to demonstrate better. The inheritance of your example isn't
    > the difficulty. It's not class to class inheritance that makes things
    > tricky, it's the modules. Which is what leads us to use ClassMethods
    > modules and included callbacks. Generally, the more meta-code one must
    > use (or just plan code for that matter) the more fragile a solution.
    > And really, when we are depending on a callback to achieve a common
    > pattern, that's a sure sign that something is amiss.
    >
    > > > Lastly, you are missing the point when you
    > > > you say, "prefer to define modules explicitly... that's a cleaner way
    > > > to achieve what you want". That's not the issue. Again, I encourage
    > > > you to have a look as Facets' inheritor.rb lib to get a better
    > > > understanding of this.

    > >
    > > Um, you got me stumped here: in your original post you advertised
    > > easier reuse of singleton methods by making the singleton class a
    > > singleton module instead. Where exactly am I missing the point?

    >
    > Because it's not a matter of preference. There is no choice. The only
    > way to currently achieve class-level class->module->class inheritance
    > is through secondary modules and meta-coding. And I certainly don't
    > see how being required to create an artificial abstraction (for
    > example, ClassMethods) leads to cleaner code.
    >
    > Besides, no amount of meta-coding and secondary modules will allow for
    > "fluent singleton notation" at all. Eg.
    >
    > def memoized.foo
    > ...
    > end


    Thanks for the explanation! I think I got a better picture now.

    Another side note: I thought I remember there was a way to get two
    instances with the same singleton class but apparently I cannot
    reproduce it. Even if, that case was probably so esoteric that it is
    no obstacle to changing singleton classes to singleton modules. (And
    while that happens we should also introduce a method to access it.)

    Kind regards

    robert
     
    Robert Klemme, Sep 14, 2007
    #13
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