Const_defined? not quite in sync with const_get?

Ruby
1

Assuming the following snippet, I’d expect Foo::const_defined?(:Bar)
to be routed via Foo::const_missing. Instead it resolves to top-level
Bar. Can someone explain why please?

–code–

class Bar
end

module Foo
def self.const_missing(konst)
puts “missing konst #{konst}”
end
end

Foo::Bar
#=> missing konst Bar # Unsurprising behaviour

p Foo::const_defined?(:Bar)
false # Unsurprising behaviour

p Foo::const_get(:Bar)
#=> Bar # Surprising behaviour

–/code–

Cheers,

Chris

2

non-prefixed constants follow the normal scoping rules:

Thanks for the clarification. Purely out of interest, do you find
this behaviour a bit odd (or am I missing obvious reasons as to why it
is implemented this way)?

Chris

3

On Thu, 21 Sep 2006, Chris R. wrote:

def self.const_missing(konst)
p Foo::const_get(:Bar)
#=> Bar # Surprising behaviour

–/code–

Cheers,

Chris

non-prefixed constants follow the normal scoping rules:

harp:~ > cat a.rb
class Bar
end

module Foo
def self.const_missing(konst)
puts “missing konst #{konst}”
end
end

Foo::Bar
#=> missing konst Bar # Unsurprising behaviour

p Foo::const_defined?(‘Bar’)
false # Unsurprising behaviour

p Foo::const_get(‘Foo::Bar’)

^^^^^

^^^^^

^^^^^

#=> Bar # Surprising behaviour

harp:~ > ruby a.rb
missing konst Bar
false
a.rb:16:in `const_get’: wrong constant name Foo::Bar (NameError)
from a.rb:16

-a

4

class C
a = const_get(‘Array’).new
end

behave

i agree it can be confusing, but the alternative, requiring all constants to
be fully scoped, would be an enormous pain in the ass.

Ah, of course, that makes sense. Cheers.

5

On Thu, 21 Sep 2006, Chris R. wrote:

non-prefixed constants follow the normal scoping rules:

Thanks for the clarification. Purely out of interest, do you find
this behaviour a bit odd (or am I missing obvious reasons as to why it
is implemented this way)?

Chris

so that

class C
a = Array.new # up scope
end

and

class C
a = const_get(‘Array’).new
end

behave

i agree it can be confusing, but the alternative, requiring all
constants to
be fully scoped, would be an enormous pain in the ass.

cheers.

-a

6

Date: Thu, 21 Sep 2006 02:06:56 +0900
Posted: Wed, 20 Sep 2006 18:06:54 +0100
From: “Chris R.” [email protected]
Reply-To: [email protected]
Subject: const_defined? not quite in sync with const_get ??
To: [email protected] (ruby-talk ML)
Message-Id:
[email protected]
X-ML-Name: ruby-talk
X-Mail-Count: 215393

2006/9/21, Chris R. [email protected]:

def self.const_missing(konst)
p Foo::const_get(:Bar)
#=> Bar # Surprising behaviour

–/code–

Cheers,

Chris

–
e$B$J$+$@$G$9!#e(B

7

Hi,

Sorry for mispost.

At Thu, 21 Sep 2006 02:06:56 +0900,
Chris R. wrote in [ruby-talk:215393]:

Foo::Bar
#=> missing konst Bar # Unsurprising behaviour

p Foo::const_defined?(:Bar)
false # Unsurprising behaviour

p Foo::const_get(:Bar)
#=> Bar # Surprising behaviour

It has changed in 1.9.

p Foo::const_defined?(:Bar) #=> true

p Foo::const_get(:Bar) #=> Bar

p Foo::const_defined?(:Bar, false) #=> false

p Foo::const_get(:Bar, false) #=> missing konst Bar