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comp.lang.ruby

Enumerating constants in anonymous modules

Xavier Shay

10/9/2008 3:22:00 PM

How can I enumerate all the constants inside the module in the second
example?
Notice that #constants array is empty

For bonus points, which is the "correct" eval method to use to define a
class inside the module (they all seem to work)

Cheers,
Xavier

module Foo
class A
end
end

puts Foo.constants.inspect # => ["A"]

[:module_eval, :instance_eval, :class_eval].each do |method|
m = Module.new
m.send(method) do
class B
end
end
puts m.constants.inspect # => [] (I would expect ['B'])
puts m.const_get('B').inspect # => B
end
--
Posted via http://www.ruby-....
2 Answers

David A. Black

10/9/2008 3:55:00 PM

0

Hi --

On Fri, 10 Oct 2008, Xavier Shay wrote:

> How can I enumerate all the constants inside the module in the second
> example?
> Notice that #constants array is empty

See comments in code, below.

> For bonus points, which is the "correct" eval method to use to define a
> class inside the module (they all seem to work)

module_eval and class_eval are synonyms for each other. I'd use
module_eval over instance_eval, unless you specifically need instance
methods to be singleton methods on the module object (which is
unlikely, and can be achieved by other means under module_eval).

> module Foo
> class A
> end
> end
>
> puts Foo.constants.inspect # => ["A"]
>
> [:module_eval, :instance_eval, :class_eval].each do |method|
> m = Module.new
> m.send(method) do
> class B

The constant B is resolved in the usual "semi-static", eager way,
which means it does not belong to the new module: it's actually being
defined at the top level.

> end
> end
> puts m.constants.inspect # => [] (I would expect ['B'])
> puts m.const_get('B').inspect # => B
> end

Try this:

m = Module.new
m.module_eval do
class self::B # forces Ruby to wait to bind the constant
end
end
puts m.constants.inspect # ["B"]
puts m.const_get('B').inspect # #<Module:0x220164>::B


David

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Xavier Shay

10/9/2008 4:04:00 PM

0

David A. Black wrote:
> Hi --
>
> On Fri, 10 Oct 2008, Xavier Shay wrote:
>
>> How can I enumerate all the constants inside the module in the second
>> example?
>> Notice that #constants array is empty
>
> See comments in code, below.
>
>> For bonus points, which is the "correct" eval method to use to define a
>> class inside the module (they all seem to work)
>
> module_eval and class_eval are synonyms for each other. I'd use
> module_eval over instance_eval, unless you specifically need instance
> methods to be singleton methods on the module object (which is
> unlikely, and can be achieved by other means under module_eval).

I'm not really sure what I need, I'll do some more reading and come back
if I have any further questions. Using module_eval for now.

> Try this:
>
> m = Module.new
> m.module_eval do
> class self::B # forces Ruby to wait to bind the constant
> end
> end
> puts m.constants.inspect # ["B"]
> puts m.const_get('B').inspect # #<Module:0x220164>::B

This does what I want, cheers
--
Posted via http://www.ruby-....