Historical errata for Metaprogramming Ruby 2

“a ||= []” should be “a || (a = [])” instead of “a = (a || [])”.

ref. www.ruby-doc.org/core-2.1.1/doc/syntax/assignment_rdoc.html
> Note that these two operators behave more like a || a = 0 than a = a || 0.

“At the top level of your program, the current class is Object, the class of main. (That’s why, if you define a method at the top level, that method becomes an instance method of Object).”

As far as I can see it is not an instance method of Object but a private method:

2.1.1 :001 > self
=> main
2.1.1 :002 > self.class
=> Object
2.1.1 :003 > def new_method;p ‘Hey there’; end
=> :new_method
2.1.1 :004 > Array.new.new_method
NoMethodError: private method `new_method’ called for []:Array
\tfrom (irb):4
\tfrom /Users/brunodp/.rvm/rubies/ruby-2.1.1/bin/irb:11:in `

If I run the example on this page, I get the error that Money.from_numeric is undefined - I have installed the current version (6.1.1) of the money gem.

The example uses Test::Unit. Test::Unit is currently replaced by MiniTest (I get an error stating the same when i try to run with the example code in Ruby 2.1) - perhaps use MiniTest instead?

to_money is deprecated in the gem money v6.1 - see money-rails/issues/150 on github. I get error “undefined method `to_money’ for 100:Fixnum (NoMethodError)” when I run the sample on this page.

“I want to open this object, and I don’t particularly care it’s a class,”

Missing an ‘if’ => Should be “…if it’s a…”

The paragraph says:
“While talking about superclasses, you can ask ourselves one more question. What is the superclass of Class?”

I think it should say WE can ask ourselves.

A variable can references a class just like any other object:

I think it should be REFERENCE instead of referenceS

The code from example money_example.rb is deprecated, the money gem has bin split into money and monetize - method from_numeric must be used from Monetize module. Here is my fix (also removed second require ‘money’): GIST: a9f2bd6492ce857c3f80

Best Regards
Bart

The sentence: “if the validation fails, and otherwise falls back to the original save_with_validation!.”
I think should be: “if the validation fails, and otherwise falls back to the original save_withOUT_validation!.”

The last paragraph says: “Validation#save_with_validation
returns false if the validation fails, or if the caller explicitly disables validation.
Otherwise, it just calls the original save_without_validation.”

But based on the validation code:

“if perform_validation && valid? || !perform_validation”

I think the method:
“save_with_validation(perform_validation = true)”

will still call the original save_without_validation method if the caller explicitly disables validation since the call:

save_with_validation(false) will make !perform_validation
evaluate as TRUE (unless I am misunderstanding the sentence “he caller explicitly disables validation”)

I have a question about method lookup.
You have said “one step to the right, then up”, I think it is not exactly.
If the object is a instance of Class, when i call a class method it will find in itself and its superclasses, and then goto its class.
e.g.:
class A
def self.method_a
p ‘method_a in A’
end
end

class B < A
end

class Class
def method_a
p ‘method_a in Class’
end
end

B.method_a #=> “method_a in A”

So, i think it should be “Goes up, the one step to the right”.

It says to install `test-unit` for ruby 2.2 or later. Isn’t that version (2.2) a typo? We are on 2.1.

For the refinement gotchas,

using MyClassRefinement
MyClass.new.my_method

returns “original my_method()” for me instead of the “refined my_method()”

Refinement does not seem to work for me. I am using ruby 2.1.1

On 3rd paragraph:

“(We discussed additional arguments to method_eval, such as FILE and LINE, in The irb Example, on page 144.)”

method_eval should be module_eval

class_eval is used in an example, but it’s only introduced later (on page 107).

The text should be updated for JRuby 9000 (“supporting Ruby 2.2, except for Refinements and a few other things”).

From Kado: “blocks/using.rb” on ch04 doesn’t work because Ruby 2.x has “using” method for Refinements.

ch03: There are “accounting” department and “purchasing” department are intermingled in the same context.

Introduction : “Until Ruby 2.1, test-unit was a standard library. From Ruby 2.2 onward, you need to install it as a gem” isn’t correct. 2.1 doesn’t have it (actually it is mini-test) and 2.2 has real test-unit.

From Kado: “blocks/flat_scope_3.rb” on ch04 has a unwanted assertion method. It seems a test code for your self. And it needs two ‘puts’ in front of the strings.

In the sentence “The only the first two methods include the new feature.” there is an extra “The”.
“The only the first two methods include the new feature.” => “Only the first two methods include the new feature.”

“Try defining a method inside another method with def, and you’ll see that the new method is defined on the class of self.” Not 100% accurate - in case self belongs to a subclass, the method is defined in the class of the wrapper method.

From Kado: Ch05: “Kernel#callers” => “Kernel#caller”.

In sentence “and extended defines an @_dependenciesClass Instance Variable (109) on the includer” , should includer be extender?

“+ Here’s how you create a Money object from a number:”

It has still “Money” string.

Ch04 about Padrino: “written with the RSpec test gem”

It is not RSpec but minitest.
ref. github.com/padrino/padrino-framework/blob/0.11.3/padrino-core/test/mini_shoulda.rb

From Kado:
ch09 The Validations Modules
Both ActiveModel::Validation and ActiveRecord::Validation misses validation[s].

On Stack Overflow: questions/29980160/metaprogramming-ruby-2-example-from-the-book-isnt-working-when-i-try-it-troubl

[09/06/15 15:21:19] KADO Masanori: ch02: monetise gem doesn’t work on Ruby 2.2.2.
see https: teamtreehouse.com/forum/using-the-gem-money-it-will-not-allow-me-to-use-puts-price
[09/06/15 15:21:54] KADO Masanori: It needs to insert " I18n.enforce_available_locales = false" after ‘require “monetise”’
[09/06/15 15:41:33] KADO Masanori: It also needs “require ”forwardable“” before “require ”monetize“”
——
[14/06/15 16:37:34] KADO Masanori: monetize:
All versions don’t work, even github master … but it might be fixed in the future version. So It seems better to use a footnote if you don’t want to change the sample code (I also like current simple code).
[14/06/15 16:41:05] KADO Masanori: Like this (of course in Japanese) : If you encountered ``uninitialized constant Monetize::Collection::Forwardable (NameError)``, insert ``require “forwardable”`` into the top of the code. If you encountered ``enforce_available_locales!’: :en is not a valid locale (I18n::InvalidLocale)``, insert ``I18n.enforce_available_locales = false`` after ``require “monetize”``.

appendix01: changed sample code at the point of ‘*’.

class MyClass

• private :my_attribute=
  end

obj.set_attribute 11 # No error!

• obj.my_attribute # => 11

> “revert()” to explore_array, the method will evaluate the string “[’a’, ’b’, ’c’].revert()”. It’s time to try out this utility:

revert() should be reverse()

Kado rightly observers that “the & operator” is a misnomer. It’s not technically an operator.

Some people call it “the & modifier”, but I’m not sure how common this is in English. Check it, and decide whether to add a one-sentence note to the book, or even change the name altogether.

The last two lines in the “methods/dynamic_definition.rb” code sample are tests, and they shouldn’t appear in the text.

The writer is describing the “save_with_validation!” and mistakenly reuse the name again when what he meant was “save_without_validation!”.
This is how it appears in the document -> “The actual validation happens in the valid? method. Validation#save_with_validation returns false if the validation fails, or if the caller explicitly disables validation. Otherwise, it just calls the original save_without_validation. Validation#save_with_validation! raises an exception if the validation fails, and otherwise falls back to the original save_with_validation!.”

In the Safari Books Online edition (no page numbers available), Chapter 3, Quiz: Bug Hunt, the sample output is:
5…​
 6…
​  10…
​  Bob got a 3​
 7…​
 4…
​  3…
​  Frank got a 10
(swap Bob’s and Frank’s lines). In the paragraph preceding the last listing in the same section, should be “Bill’s team”, not Frank’s.

The Pry example is not behaving as advertised. Pry instances do not have a quiet= method, nor do they have a refresh method. Is there a specific version of Pry you are using for these examples?

The example regarding private method calls no longer works on ruby 2.7. Looks like it’s caused by Ruby issue number 16783 (which I would link to, but this form doesn’t accept links)