From: Walton Hoops <walton@...>
Date: 2009-12-02T02:38:35+09:00
Subject: Re: Ruby should support consistent constant lookup (and dynamic)

> -----Original Message-----
> From: alexandre_mutel@yahoo.fr [mailto:alexandre_mutel@yahoo.fr]
> Just a general remark about this problem. Shouldn't Ruby have a builtin
> mechanism to resolve dynamically and efficiently constants handling
> lexical scopes correctly?
> 
> Seems that in 1.9, we have this features:
> irb(main):001:0> class A
> irb(main):002:1>   BAR = 1
> irb(main):003:1>   class B
> irb(main):004:2>     def self.foo(&b)
> irb(main):005:3>       instance_eval(&b)
> irb(main):006:3>     end
> irb(main):007:2>   end
> irb(main):008:1> end
> => nil
> irb(main):009:0> A::B.foo do
> irb(main):010:1* puts BAR
> irb(main):011:1> end
> 1
> => nil
> irb(main):012:0>
> 
> BUT now, try to do it with const_get, and we got a NameError:
> irb(main):012:0> A::B.foo do
> irb(main):013:1* puts const_get(:BAR)
> irb(main):014:1> end
> NameError: uninitialized constant A::B::BAR
>         from (irb):13:in `const_get'
>         from (irb):13:in `block in irb_binding'
>         from (irb):5:in `instance_eval'
>         from (irb):5:in `foo'
>         from (irb):12
>         from E:/Code/Ruby/bin/irb:12:in `<main>'
> 
> This is quite frustrating when writing a DSL language to not being able
> to resolve constants dynamically (without getting a costly module
> hierarchy and checking the constant in each module...)

irb(main):001:0> class A
irb(main):002:1>   BAR=1
irb(main):003:1>   class B
irb(main):004:2>    def self.foo(&b)
irb(main):005:3>      instance_eval(&b)
irb(main):006:3>    end
irb(main):007:2>   end
irb(main):008:1> end
=> nil
irb(main):012:0> A::B.foo do
irb(main):013:1* puts eval('BAR')
irb(main):014:1> end
1
=> nil

Done.  A more experienced Rubist may be able to do it without eval, but
that works.

> Should'nt this constant lookup be consistent between static and
> const_get?

No, that's not what const_get is for.  It's operating on a class
not a scope.  That's why it's an instance method.  The way you
calling it is literally equivalent too: 

A::B.const_get(:BAR)

In other words, you are asking for _B's_ constants.  Since B,
nor any of its ancestors have the constant BAR, an error occurs.

Maybe this demonstrates more clearly what I mean:

irb(main):001:0> class A
irb(main):002:1>   BAR=1
irb(main):003:1>   def self.BAR
irb(main):004:2>     BAR
irb(main):005:2>   end
irb(main):006:1>   class B
irb(main):007:2>   end
irb(main):008:1> end
=> nil
irb(main):009:0> A.BAR
=> 1
irb(main):011:0> A::B.BAR
NoMethodError: undefined method `BAR' for A::B:Class
        from (irb):11
        from C:/Ruby19/bin/irb:12:in `<main>'
irb(main):012:0> A.const_get :BAR
=> 1
irb(main):013:0> A::B.const_get :BAR
NameError: uninitialized constant A::B::BAR
        from (irb):13:in `const_get'
        from (irb):13
        from C:/Ruby19/bin/irb:12:in `<main>'
irb(main):014:0>

Notice, const_get works the same as the accessor method 
for BAR.