Const_missing - why is it necessary to hook it at Module?

ruby-talk
1

Why doesn't the following code work?

class Foo
  def Foo.const_missing(sym)
    p sym
    sym.to_s
  end
end

Foo.new.instance_eval {
  Bar
}

I thought the missing constant Bar would be caught
by the const_missing, since it's available on the
class whose instance is being instance_eval'd.

What gives? I'm writing a DSL and want to catch missing
constants within a block being instance_eval'd

Clifford Heath.

2

Hi --

Why doesn't the following code work?

class Foo
        def Foo.const_missing(sym)
                p sym
                sym.to_s
        end
end

Foo.new.instance_eval {
        Bar
}

I thought the missing constant Bar would be caught
by the const_missing, since it's available on the
class whose instance is being instance_eval'd.

What gives? I'm writing a DSL and want to catch missing
constants within a block being instance_eval'd

All that instance_eval does is set self and execute the block.
Constants don't depend on self for their scope; they use a kind of
quasi-static scoping, and the Bar in your block is resolved
(unsuccessfully) as Bar from the top level.

You'd need to add your const_missing method to Object, so as to cover
the top level.

David

···

On 3/18/07, Clifford Heath <no.spam@please.net> wrote:

--
Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black\)
   (See what readers are saying! http://www.rubypal.com/r4rrevs.pdf\)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

3

David A. Black wrote:

Constants don't depend on self for their scope; they use a kind of
quasi-static scoping

Ok, thanks David, I figured it was something like that.
There are very few cases where const_missing gets used
(Rails and Rake being two), and it's not obvious from
them or from the documentation that it works like that.

BTW, I recently saw a video of your "Curious developer"
talk on database design at the RailsConf 2006. I expect
to address many of the issues through my "ActiveFacts"
project (this DSL I'm working on is a fact-based data
modeling language). Would you be interested in private
communication regarding this project?

Clifford Heath.

4

Clifford Heath wrote:

Would you be interested in private
communication regarding this project?

David, I got your response. Perhaps you didn't see mine?

Clifford Heath.