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comp.lang.ruby

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

Clifford Heath

3/18/2007 6:56:00 AM

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.
3 Answers

dblack

3/18/2007 12:14:00 PM

0

Hi --

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

--
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A. RUBY FOR RAILS by David A. Black (http://www.manning...)
(See what readers are saying! http://www.r.../r...)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.r...)

Clifford Heath

3/18/2007 11:58:00 PM

0

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.

Clifford Heath

3/20/2007 3:17:00 AM

0

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.