Issue #11665 has been updated by Daniel P. Clark.


Oddly there are some people who've found the dynamic defining of methods this way useful as a state machine. This recent blog post demonstrates it http://weblog.jamisbuck.org/2015/10/17/dynamic-def.html

I inclined to agree with you Keith about the added compounding of complexity and the potential side effects of lambdas.  After thinking about this it sound like what you're looking for is a lot like refinements brought down from class level into method definitions.  Nobuyoshi has written an excellent example with #6 .  If that could be implemented in a meta-programming way as a method on Class, maybe as `scoped_def :m, *a, &b` you can use it anywhere.

Here's an example without using refinements

~~~ruby
class A
  def example(x)

    class << self
      private def hello
        "hello"
      end
    end

    x.call(self)

  ensure # if proc call above fails we don't want to leak the method
    class << self
      undef :hello
    end

  end
end

a = A.new

a.example ->i{ puts i.send :hello}
#hello
# => nil 

a.example ->i{ puts i.hello}
#NoMethodError: private method `hello' called for #<A:0x000000012486e0>

a.send :hello
#NoMethodError: undefined method `hello' for #<A:0x000000012486e0>
~~~

So having something to scope methods defined within a method could be as simple as an `undef` at the end of your scope.  I liked using a private method approach above which can only be defined for the singleton instance... but if you weren't that worried about it being a private method "during its execution" then you could use just `undef`.

~~~ruby
class B
  def example2(x)
    def hello2
      "hello2"
    end
    x.call(self)
    ensure undef :hello2
  end
end


b = B.new

b.example2 ->i{puts i.send :hello2}
hello2
# => nil 

b.example2 ->i{puts i.hello2}
#hello2
# => nil 

b.hello2
#NoMethodError: undefined method `hello2' for #<B:0x0000000128a928>
~~~

----------------------------------------
Feature #11665: Support nested functions for better code organization
https://bugs.ruby-lang.org/issues/11665#change-54821

* Author: Keith Bennett
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
The wisdom of using local variables has been internalized in all of us from the beginning of our software careers.  If we need a variable referring to data that is used only in a single method, we create a local variable for it.

Yet if it is logic to which we need to refer, we make it an instance method instead.

In my opinion, this is inconsistent and unfortunate. The result is a bloated set of instance methods that the reader must wade through to mentally parse the class.  The fact that some of these methods are used only by one other method is never communicated by the code; the reader has to discover that for him/herself.

The number of possible interactions among the instance methods is one of many measures of our software's complexity.  The number of possible instance method interactions is <code>(method_count * (method_count) - 1)</code>.  Using this formula, a class with 10 methods will have a complexity of 90.  If 4 of those methods are used by only 1 other method, and we could move them inside those methods, the complexity would plummet to 30 <code>(6 * (6 - 1))</code>, a third of the original amount!

While it is possible to extract subsets of these methods into new smaller classes, this is not always practical, especially in the case of methods called only by the constructor.

Fortunately, we do have lambdas in Ruby, so I will sometimes create lambdas inside methods for this purpose.  However, lambdas are not as isolated as methods, in that they can access and modify local variables previously defined outside their scope.  Furthermore, the lambdas can be passed elsewhere in the program and modify those locals from afar! So using methods would be cleaner and safer.

Another weakness of using lambdas for this purpose is that, unlike methods that are created at interpret time, lambdas are objects created at runtime -- so if a method creating 2 lambdas is called a million times in a loop, you'll need to create and garbage collect another 2 million objects. (This can be circumvented by defining the lambdas as class constants or assigning them to instance variables, but then they might as well be instance methods.)

I realize that implementing this feature would be a substantial undertaking and may not be feasible at this time. That said, I think it would be useful to discuss this now so we might benefit from its implementation someday.

* * * *

(Much of this content is communicated in my talk on Ruby lambdas; slide show is at https://speakerdeck.com/keithrbennett/ruby-lambdas-functional-conf-bangalore-oct-2014 and YouTube video of the presentation at FunctionalConf in Bangalore at https://www.youtube.com/watch?v=hyRgf6Qc5pw.)

Also, this post is also posted as a blog article at http://www.bbs-software.com/blog/2015/11/07/the-case-for-nested-methods-in-ruby/.



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