From: Austin Ziegler <halostatue@...>
Date: 2006-03-13T05:48:49+09:00
Subject: Re: [RCR] abstract method in Ruby

On 3/12/06, kwatch <kwa@kuwata-lab.com> wrote:> Logan,>> Oh look, this code already raises an exception, a NoMethodError. Now>> I know that to include Enumerable I have to have an each method.>> That's all there is to it.> I think it is very important that the code itself is descriptive. The> following code doesn't describe that method each() is abstract.
>   module Enumerable>     def map>       arr = []>       each { |elem| arr << yield(elem) }>       arr>     end>   end
Actually, the method #each is not abstract in Enumerable. It's aprerequisite. There's a difference. In the C++ and Java world, it's*necessary* to define a prerequisite method as an abstract method,because without a method definition, the compiler goes stupid and can'tfind anything and won't then compile your code at all. Ruby sees thecall to #each with a block and says "okay, I'll trust that you'vesatisified this until I hit it at run time."
> The following code itself describes that method each() is abstract.> It's more descriptive.
>   module Enumerable>     abstract_method :each>     def map>       arr = []>       each { |elem| arr << yield(elem) }>       arr>     end>   end
> Please take care of reading code, as well as writing code. You may> insist that documentation is sufficient to describe that, but> documentation is assistant and code should be prime.
What you've got is no better than:
  # Every method provided in Enumerable requires that the including  # class defines an #each method that iterates one item at a time.  module Enumerable    def map      arr = []      each { |elem| arr << yield(elem) }      arr    end  end
Except that it actually adds a little code for what is essentially adocumentation annotation. As I said, it adds little to no value to Rubyand IMO doesn't belong in the core.
>> Oh look, this code already raises an exception, a NoMethodError.> NoMethodError is not proper to abstract method, I think.
I think that it is. It says that there's no method for what's beinglooked for. Having "abstract_method :each" doesn't add value to that.
>> Look at your Visitor example for instance,>> in ruby there's not even a need for the Visitor  module, all you have>> to do is implement visit_foo and visit_bar in MyVisitor. In ruby, no>> one cares who your parents were, all they care about is if you know>> what you are talking about.>> Assume that you need to define many visitor classes.>>   module Visitor>     abstract_method :visit_foo, :visit_bar>   end>>   class Hoge>     include Visitor>     def visit_foo; ... ; end>     def visit_bar; ... ; end>   end>>   class Fuga>     include Visitor>     def visit_foo; ... ; end>     def visit_bar; ... ; end>   end>>   class Geji>     include Visitor>     def visit_foo; ... ; end>     def visit_bar; ... ; end>   end>> The above code itself describes that "class Hoge, Fuga, and Geji> are visotr classes" very clearly.> 'Visitor' module may not be necessary when a number of visitor class> is only 1.> But there should be 'Visitor' module in the case that many visitor> classes are needed. It's more descriptive.
Disagree. In Ruby, inclusion of a Module means something. Specifically,it means that you're mixing in functionality ... not that you're mixingin warnings or merely documenting things.
    # Items of class Hoge can be visited by classes that know how to    # #visit_foo or #visit_bar.    class Hoge      def visit_foo; ... ; end      def visit_bar; ... ; end    end
    # Items of class Fuga can be visited by classes that know how to    # #visit_foo or #visit_bar.    class Fuga      def visit_foo; ... ; end      def visit_bar; ... ; end    end
    # Items of class Geji can only be visited by classes that know how    # to #visit_foo.    class Geji      def visit_foo; ... ; end    end
>> Start thinking like a duck man.> You mean 'duck typing'? Duck typing is one thing and abstract method> is another.
Not really. As I noted earlier, abstract methods are necessary instatically typed and linked languages because they're aggressive inresolving "facts", whereas Ruby is lazy and doesn't try to resolvethings until they're needed.
>> Ask yourself what is the purpose of an abstract method.>> Then ask yourself if that need is already fufilled in ruby.> Abstract method is useful to realize 'descriptive code'.> It increases code maintanancability.
Disagree. IME, it does the opposite and reduces the descriptiveness andusefulness of the code. Stop thinking in C++ -- in Ruby, abstractmethods simply aren't necessary.
Trust me. You won't find them in Rails, PDF::Writer, or any of a varietyof other large and popular packages.
-austin--Austin Ziegler * halostatue@gmail.com               * Alternate: austin@halostatue.ca