Understanding private methods in Ruby - ruby

Question

class Example
private
def example_test
puts 'Hello'
end
end
e = Example.new
e.example_test
This of course will not work, because we specified explicit receiver - instance of Example (e), and that is against a "private rule".
But I cannot understand, why one cannot do in Ruby this:
class Foo
def public_m
self.private_m # <=
end
private
def private_m
puts 'Hello'
end
end
Foo.new.public_m
The current object inside public_m method definition (i.e. self) is the instance of Foo. So why it is not allowed? To fix that I have to change self.private_m to just private_m. But why this differ, isn't the self an instance of Foo inside public_m? And who is the receiver of bare-word private_m call? Isn't that self - what actually you omit because, Ruby will do it for you (will call private_m on self)?
I hope I didn't confuse it too much, I am still fresh to Ruby.
EDIT:
Thank you for all the answers. Putting them all together I was able (finally) to grok the obvious (and not so obvious for someone, who have never seen things like Ruby): that self itself can be
explicit and implicit receiver and that make the difference. So there are two rules, if you want to call a private method: self must be implicit receiver, and that self must be an instance of current class (Example in that case - and that takes place only when self if inside instance method definition, during this method execution). Please correct me if I am wrong.
class Example
# self as an explicit receiver (will throw an error)
def explicit
self.some_private_method
end
# self as an implicit receiver (will be ok)
def implicit
some_private_method
end
private
def some_private_method; end
end
Example.new.implicit
Message for anyone who could find this question during the google trails: this may be helpful - http://weblog.jamisbuck.org/2007/2/23/method-visibility-in-ruby

Answer 1

Here's the short and the long of it. What private means in Ruby is a method cannot be called with an explicit receivers, e.g. some_instance.private_method(value). So even though the implicit receiver is self, in your example you explicitly use self so the private methods are not accessible.
Think of it this way, would you expect to be able to call a private method using a variable that you have assigned to an instance of a class? No. Self is a variable so it has to follow the same rules. However when you just call the method inside the instance then it works as expected because you aren't explicitly declaring the receiver.
Ruby being what it is you actually can call private methods using instance_eval:
class Foo
private
def bar(value)
puts "value = #{value}"
end
end
f = Foo.new
begin
f.bar("This won't work")
rescue Exception=>e
puts "That didn't work: #{e}"
end
f.instance_eval{ bar("But this does") }
Hope that's a little more clear.
-- edit --
I'm assuming you knew this will work:
class Foo
def public_m
private_m # Removed self.
end
private
def private_m
puts 'Hello'
end
end
Foo.new.public_m

Answer 2

The definition of private in Ruby is "can only be called without an explicit receiver". And that's why you can only call private methods without an explicit receiver. There is no other explanation.
Note that there actually is an exception to the rule: because of the ambiguity between local variables and method calls, the following will always be resolved to be an assignment to a local variable:
foo = :bar
So, what do you do if you want to call a writer called foo=? Well, you have to add an explicit receiver, because without the receiver Ruby simply won't know that you want to call the method foo= instead of assigning to the local variable foo:
self.foo = :bar
But what do you do if you want to call a private writer called foo=? You can't write self.foo = because foo= is private and thus cannot be called with an explicit receiver. Well, actually for this specific case (and this case alone), you can actually use an explicit receiver of self to call a private writer.

Answer 3

It's weird, but many things about Ruby's visibility modifiers are weird. Even if self is the implicit receiver, actually spelling it out makes it explicit in the eyes of the Ruby runtime. When it says that private methods cannot be called with an explicit receiver, that is what it means, even self counts.

Answer 4

IIRC, private methods allow only implicit receiver (which is always self, of course).

Answer 5

Sorry for my prevoius answer. I just don't understand your question.
I changed your code like this:
class Foo
def public_m
private_m # <=
end
def Foo.static_m
puts "static"
end
def self.static2_m
puts "static 2"
end
private
def private_m
puts 'Hello'
end
end
Foo.new.public_m
Foo.static_m
Foo.static2_m
Here is a call of instance method:
def public_m
private_m # <=
end
Here are a call of class methods:
def Foo.static_m
puts "static"
end
def self.static2_m
puts "static 2"
end
Foo.static_m
Foo.static2_m

Answer 6

Adding some enhancements to User Gates solution. Calling a private method to class method or an instance method is pretty much possible. Here is the Code Snippets. But not recommended.
Class Method
class Example
def public_m
Example.new.send(:private_m)
end
private
def private_m
puts 'Hello'
end
end
e = Example.new.public_m
Instance Method
class Example
def self.public_m
Example.new.send(:private_m)
end
private
def private_m
puts 'Hello'
end
end
e = Example.public_m

Answer 7

Does not exactly answer the Question, but you can call private methods this way
class Example
private
def example_test
puts 'Hello'
end
end
e = Example.new
e.send(:example_test)

Answer 8

Just in case someone stumbles this now. From Ruby 2.7, calling a private method with a literal self as the receiver is now allowed.
We can verify this as well by running the original ruby snippet against versions 2.6.9 and 3.1.
(ins)tmp->cat sample.rb
class Foo
def public_m
self.private_m # <=
end
private
def private_m
puts 'Hello'
end
end
Foo.new.public_m
# See no exception is raise with 3.1 version
(ins)tmp->ruby -v
ruby 3.1.0p0 (2021-12-25 revision fb4df44d16) [x86_64-darwin20]
(ins)tmp->ruby sample.rb
Hello
Now if we try to run the same script with version 2.6.9, we see that exception is raised.
(ins)tmp->ruby -v
ruby 2.6.9p207 (2021-11-24 revision 67954) [x86_64-darwin20]
(ins)tmp->
(ins)tmp->ruby sample.rb
sample.rb:3:in `public_m': private method `private_m' called for #<Foo:0x00007ff95289f870> (NoMethodError)
Did you mean? private_methods
from sample.rb:11:in `<main>'