From: Robert Klemme <shortcutter@...>
Date: 2011-03-28T05:15:26+09:00
Subject: Re: Dynamic classes

On 26.03.2011 19:16, Vicente Bosch Campos wrote:
>
>> On 26.03.2011 15:38, Vicente Bosch Campos wrote:
>>> and I should be able to change its properties and that should be
>>> independent of having the definition assigned to a variable
>>> otherwise it would be discarded as you say.
>>
>> You can change a class's properties, e.g. by adding methods etc.
>> It's just that the name happens to be a read only property and
>> there is some additional magic which will set it on first constant
>> assignment.  I don't see how that collides with a "purist
>> perspective".
>
> I have not explained my self properly. Imagine we are talking about
> making an instance of any other object e.g. a Car and the car has a
> name and it allows you reference the car instance by it in the
> future.
>
> Would you prefer the car name is defined by the variable that
> contains it or by setting the name in the constructor?
>
> hotwheels = Car.new
>
> current_car = Car.new("hotwheels")
>
> I don't think anyone would understand directly that I have
> automatically set a property inside my car with the variable name. It
> would be a bit counterintuitive and different from how any other
> instance properties work on this class and how they are usually set
> on any other type of object of any library.
>
> Considering the Class to be an object too by not allowing me set the
> name at the .new and taking the name of the left hand side of the
> equality sign in case it is a Constant ( otherwise it does not) it is
> not acting very much like any other object.

OK, now I get your point.  Thanks!  I think Matz just decides that doing

X = Class.new do
   def foo; end
end

is less clean than

class X
   def foo; end
end

and hence created the const magic because class definitions are so 
common.  If you want, you can still use the bar bones idiom though.

>> The approach with Object.const_set basically only makes sense if
>> the name is dynamic, too.
>
> Yes sorry, I do want to have the name dynamic too ( I might be going
> off-topic for the original post, sorry).

Then consider this: if the name is dynamic during definition it 
certainly has to be dynamic during referencing as well.  In that case 
there is probably not much point in assigning the class to a constant. 
Rather a Hash would be a much better place to keep all those classes 
around.  You could even do something like this:

irb(main):012:0> cl = Hash.new do |h,name|
irb(main):013:1* c = Class.new
irb(main):014:1> class<<c;self;end.send(:define_method, :name) { name }
irb(main):015:1> h[name]=c
irb(main):016:1> end
=> {}
irb(main):017:0> cl["Foo"]
=> #<Class:0x106c15bc>
irb(main):018:0> cl["Foo"]
=> #<Class:0x106c15bc>
irb(main):019:0> cl["Foo"].name
=> "Foo"

or

irb(main):027:0> cl = {}
=> {}
irb(main):028:0> def cl.new_class(name, &b)
irb(main):029:1> c = Class.new(&b)
irb(main):030:1> class<<c;self;end.send(:define_method, :name) { name }
irb(main):031:1> self[name]=c
irb(main):032:1> end
=> nil
irb(main):033:0> cl.new_class "Foo" do
irb(main):034:1* def bar;123;end
irb(main):035:1> end
=> #<Class:0x105a7364>
irb(main):036:0> cl["Foo"]
=> #<Class:0x105a7364>
irb(main):037:0> cl["Foo"]
=> #<Class:0x105a7364>
irb(main):038:0> cl["Foo"].name
=> "Foo"
irb(main):039:0> cl["Foo"].new.bar
=> 123

Now the Hash defines a namespace where all your dynamic classes reside. 
  If you want to get rid of them you just need to clear the Hash.  This 
approach can be extended to define a specific DSL for your domain which 
would make method definitions even simpler.

> I am just saying it would be nicer to do Class.new("Bar") would be
> much more like any other object instantiation and less questioned
> about as it would not be an exception to how the rest of the stuff
> works ( although Class object is truly a bit of an exception and its
> one of those marvelous things ruby allows us to tinker with ).

Actually I haven't seen the syntax "class Name ... end" questioned in a 
while.  I believe this is such a fundamental and frequent idiom to 
warrant a bit of syntactic sugar mixed with naming magic. :-)

> Many thanks for the healthy discussion  :-)

Ditto.

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/