classic_defined.rb

lvar = 1
defined?(lvar) # => "local-variable"
defined?(no_lvar)

$gvar = 1
defined?($gvar) # => "global-variable"
defined?($no_gvar) # => nil

CONST = 1
defined?(CONST) # => "constant"
defined?(NO_CONST)

OTHER_CONST = nil
defined?(OTHER_CONST)

module Mod; end
defined?(Mod) # => "constant"

class Klass; end
defined?(Klass) # => "constant"

module A
  class B
  end
  defined?(B) # => "constant"
end

defined?(A) # => "constant"
defined?(A::B) # => "constant"
defined?(B) # => nil

def m; end
defined?(m) # => "method"

@ivar = 1
defined?(@ivar) # => "instance-variable"
defined?(@no_ivar) # => nil

def foo?
end

defined?(foo?) # => "method"

defined?(4.+(4)) # => "method"

our_defined.rb

def our_defined?(ident, outer_binding)
  if outer_binding.local_variables.include?(ident)
    "local-variable"
  elsif global_variables.include?(ident)
    "global-variable"
  else
    binding.irb if ident == :@ivar
  end
end

lvar = 1 # => 1
our_defined?(:lvar, binding) # => "local-variable"
our_defined?(:no_lvar, binding) # => nil

$gvar = 1
our_defined?(:$gvar, binding) # => "global-variable"
our_defined?(:$no_gvar, binding) # => nil

CONST = 1
our_defined?(:CONST, binding) # => nil
our_defined?(:NO_CONST, binding) # => nil

OTHER_CONST = nil
our_defined?(:OTHER_CONST, binding) # => nil

module Mod; end
our_defined?(:Mod, binding) # => nil

class Klass; end
our_defined?(:Klass, binding) # => nil

module A
  class B
  end
  our_defined?(:B, binding) # => nil
end

our_defined?(:A, binding) # => nil
our_defined?(:"A::B", binding) # => nil

our_defined?(:B, binding) # => nil
def m; end
our_defined?(:m, binding) # => nil

@ivar = 1
our_defined?(:@ivar, binding) # => nil
our_defined?(:@no_ivar, binding) # => nil