Confusion about singleton method definition

ruby-talk
1

Hi fellow Rubists,

I'm just learning Ruby from The Well-grounded Rubist book and I cann't
understand why I cann't get the same results as in author's example.

It's about the difference between defining singleton method directly on
an object and using class << construct.
Rephrased example follows:

MYCONST=666

myobj = Object.new

class << myobj
  MYCONST=333
end

def myobj.outer_const
  puts MYCONST
end

class << myobj
  def inner_const
    puts MYCONST
  end
end

myobj.inner_const call displays 333 (singleton constant value) as
expected,
however myobj.outer_const call also displays 333 whereas it should
display the value of outer (global) MYCONST definition, ie. 666.

Did changed language definition recently somehow or is the example
and/or description in the book simply flawed ?

···

--
Posted via http://www.ruby-forum.com/.

2

The context is expected to differ from the top level inside
myobj.outer_const as well as inside instance methods of inner_const,
because they are binding-identical singleton methods. MYCONST in both
palces binds to the same `self', which is myobj =)

puts "Top level: #{self}"

myobj = Object.new

class << myobj
  def foo
    puts "class << myobj; def foo: #{self}"
  end
end

def myobj.bar
  puts "def myobj.bar: #{self}"
end

myobj.foo
myobj.bar

···

--
Posted via http://www.ruby-forum.com/.

3

Su Zhang> Thanks, but that confirms my results. The books preface states
it covers Ruby ver. 1.9.1 and I would wonder if it's valid no more for
Ruby 1.9.2 ?

···

--
Posted via http://www.ruby-forum.com/.

4

Anybody else can confirm there is no difference between constants scope
resolution at various kinds of singleton methods definiton?

···

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Posted via http://www.ruby-forum.com/.

5

Does it?

16:55:39 ~$ allruby r.rb
CYGWIN_NT-5.1 padrklemme2 1.7.7(0.230/5/3) 2010-08-31 09:58 i686 Cygwin

···

On Mon, Nov 29, 2010 at 5:30 PM, David Unric <dunric29a@gmail.com> wrote:

I'm just learning Ruby from The Well-grounded Rubist book and I cann't
understand why I cann't get the same results as in author's example.

It's about the difference between defining singleton method directly on
an object and using class << construct.
Rephrased example follows:

MYCONST=666

myobj = Object.new

class << myobj
MYCONST=333
end

def myobj.outer_const
puts MYCONST
end

class << myobj
def inner_const
puts MYCONST
end
end

myobj.inner_const call displays 333 (singleton constant value) as
expected,
however myobj.outer_const call also displays 333 whereas it should
display the value of outer (global) MYCONST definition, ie. 666.

========================================
ruby 1.8.7 (2008-08-11 patchlevel 72) [i386-cygwin]
333
666

ruby 1.9.1p430 (2010-08-16 revision 28998) [i386-cygwin]
333
666

jruby 1.4.0 (ruby 1.8.7 patchlevel 174) (2009-11-02 69fbfa3) (Java
HotSpot(TM) Client VM 1.6.0_21) [x86-java]
333
666
16:55:55 ~$ cat r.rb

MYCONST=666

myobj = Object.new

class << myobj
MYCONST=333
end

def myobj.outer_const
puts MYCONST
end

class << myobj
def inner_const
   puts MYCONST
end
end

myobj.inner_const
myobj.outer_const
16:56:57 ~$

Did changed language definition recently somehow or is the example
and/or description in the book simply flawed ?

Can you show _exactly_ the code you executed?

Kind regards

robert

--
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

6

I get that result for 1.9.2

$ rvm r.rb
1.8.7(jruby-1.5.2)
333
666
1.8.7(jruby-1.5.3)
333
666
1.9.2(macruby-0.7)
333
666
1.8.7(rbx-1.1.0-20100923)
333
666
1.8.6(ruby-1.8.6-p399)
333
666
1.8.7(ruby-1.8.7-p249)
333
666
1.8.7(ruby-1.8.7-p302)
333
666
1.9.1(ruby-1.9.1-p378)
333
666
1.9.2(ruby-1.9.2-p0)
333
333

$ cat r.rb
MYCONST=666

myobj = Object.new

class << myobj
MYCONST=333
end

def myobj.outer_const
puts MYCONST
end

class << myobj
def inner_const
  puts MYCONST
end
end

puts "#{RUBY_VERSION}(#{ENV['RUBY_VERSION']})"
myobj.inner_const
myobj.outer_const

···

On Tue, Nov 30, 2010 at 9:58 AM, Robert Klemme <shortcutter@googlemail.com>wrote:

On Mon, Nov 29, 2010 at 5:30 PM, David Unric <dunric29a@gmail.com> wrote:
> I'm just learning Ruby from The Well-grounded Rubist book and I cann't
> understand why I cann't get the same results as in author's example.
>
> It's about the difference between defining singleton method directly on
> an object and using class << construct.
> Rephrased example follows:
>
> MYCONST=666
>
> myobj = Object.new
>
> class << myobj
> MYCONST=333
> end
>
> def myobj.outer_const
> puts MYCONST
> end
>
> class << myobj
> def inner_const
> puts MYCONST
> end
> end
>
>
> myobj.inner_const call displays 333 (singleton constant value) as
> expected,
> however myobj.outer_const call also displays 333 whereas it should
> display the value of outer (global) MYCONST definition, ie. 666.

Does it?

16:55:39 ~$ allruby r.rb
CYGWIN_NT-5.1 padrklemme2 1.7.7(0.230/5/3) 2010-08-31 09:58 i686 Cygwin

ruby 1.8.7 (2008-08-11 patchlevel 72) [i386-cygwin]
333
666

ruby 1.9.1p430 (2010-08-16 revision 28998) [i386-cygwin]
333
666

jruby 1.4.0 (ruby 1.8.7 patchlevel 174) (2009-11-02 69fbfa3) (Java
HotSpot(TM) Client VM 1.6.0_21) [x86-java]
333
666
16:55:55 ~$ cat r.rb

MYCONST=666

myobj = Object.new

class << myobj
MYCONST=333
end

def myobj.outer_const
puts MYCONST
end

class << myobj
def inner_const
  puts MYCONST
end
end

myobj.inner_const
myobj.outer_const
16:56:57 ~$

> Did changed language definition recently somehow or is the example
> and/or description in the book simply flawed ?

Can you show _exactly_ the code you executed?

Kind regards

robert

--
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

7

in ruby 1.9.2, your example will provide no difference.
if you want to access the outside constant (in ruby 1.9.2), use ::MYCONST

best regards -botp

···

On Tue, Nov 30, 2010 at 8:35 PM, David Unric <dunric29a@gmail.com> wrote:

Anybody else can confirm there is no difference between constants scope
resolution at various kinds of singleton methods definiton?