About searching constant

ruby-talk
1

class A
  WHY = "I don't know why!!"

  def self.foo
    instance_eval("WHY")
  end
end

puts A.instance_eval { foo } # => I don't know why!!
puts A.instance_eval("WHY") # => uninitialized constant Class::WHY
(NameError)

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2

Weird, if you use A.instance_eval("self::WHY") it simply works... can anyone
explain it?

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On Wed, Dec 31, 2008 at 12:20 PM, Kyung won Cheon <kdream95@gmerce.co.kr>wrote:

class A
WHY = "I don't know why!!"

def self.foo
   instance_eval("WHY")
end
end

puts A.instance_eval { foo } # => I don't know why!!
puts A.instance_eval("WHY") # => uninitialized constant Class::WHY
(NameError)

###############
# Help Me^^
###############
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Posted via http://www.ruby-forum.com/\.

--
Bernardo Rufino

3

Kyung won Cheon wrote:

class A
  WHY = "I don't know why!!"

Umm ... You already posted this question, and I answered it.
Did you not like the answer?

Also as Brian mentioned, module_eval/class_eval works.

An invisible parameter is passed to a block (search for cases of
specific_eval in eval.c) which is used for constant lookups.

For A.module_eval { } and A.class_eval { }, that invisible parameter
is A. But for A.instance_eval { }, it is A's singleton class.

The constant lies in A, not in A's singleton class.

···

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4

It also works if you change instance_eval to class_eval (or
module_eval).

However I'm not entirely sure of the difference between the former and
the latter two.

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5

Bernardo Rufino wrote:

Weird, if you use A.instance_eval("self::WHY") it simply works... can
anyone
explain it?

Because that's the same as A::WHY

(i.e. inside foo.instance_eval { ... }, self is foo)

···

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6

Mike Gold wrote:

An invisible parameter is passed to a block (search for cases of
specific_eval in eval.c) which is used for constant lookups.

For A.module_eval { } and A.class_eval { }, that invisible parameter
is A. But for A.instance_eval { }, it is A's singleton class.

The constant lies in A, not in A's singleton class.

You must be inside "class
A" (or "module A") to get the constant lookup of WHY == A::WHY.

I understand it now...

Thank you!!

···

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