About instance_eval

ruby-talk
1

class A
  WHY = "I don't know why!!"

  def self.foo
    WHY
  end
end

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

# What's difference?
# Help Me^^

···

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2

Kyung won Cheon wrote:

class A
  WHY = "I don't know why!!"

  def self.foo
    WHY
  end
end

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

# What's difference?
# Help Me^^

A.instance_eval { foo } # => "I don't know why!!"
A.instance_eval { constants } # => ["WHY"]
A.instance_eval { const_get "WHY" } # => "I don't know why!!"
class A ; eval "WHY" ; end # => "I don't know why!!"

Constants follow different rules for lookup.

instance_eval changes the 'self' for method lookups, but you are still
in the top-level scope for constant lookups. You must be inside "class
A" (or "module A") to get the constant lookup of WHY == A::WHY.

Note the complementary case,

B = Class.new {
  WHEREAMI = "here"
}

WHEREAMI # => "here"

Despite being defined inside the instance of B, this constant lies in
the top-level scope.

···

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