Abstract methods in ruby?

ruby-talk
1

Hi everybody !

How do you implement something like the "abstract" keyword in Java ?
Or: Is there a technique to force a subclass to implement a special method ?

Kind regards

Dominik

2

One way is:

class Parent
   def abstract_method
     raise NotImplementedError
   end
end

You may just want to skip this step altogether though, since an exception toss is the default behavior.

Welcome to the word of dynamic typing. :wink:

James Edward Gray II

···

On Jul 11, 2007, at 5:40 PM, cypher.dp@gmail.com wrote:

How do you implement something like the "abstract" keyword in Java ?
Or: Is there a technique to force a subclass to implement a special method ?

3

cypher.dp@gmail.com wrote/schrieb <2b4059ff0707111540w6cf25d59q10944fad85bc2dd1@mail.gmail.com>:

Or: Is there a technique to force a subclass to implement a special method ?

Alternatively try something similiar to the the NVI idiom in C++:

  # interface
  module MyInterface
    def foo(*args, &block)
      do_foo(args, block)
    end
  end

  # special implementation of interface
  class MyClass
    include MyInterface

    private
    def do_foo(args, block)
      # TODO complete implementation
    end
  end

Regards
  Thomas

4

Thanks a lot for the warm welcome, James :slight_smile:

···

2007/7/12, James Edward Gray II <james@grayproductions.net>:

On Jul 11, 2007, at 5:40 PM, cypher.dp@gmail.com wrote:

> How do you implement something like the "abstract" keyword in Java ?
> Or: Is there a technique to force a subclass to implement a special
> method ?

One way is:

class Parent
   def abstract_method
     raise NotImplementedError
   end
end

You may just want to skip this step altogether though, since an
exception toss is the default behavior.

Welcome to the word of dynamic typing. :wink:

James Edward Gray II

5

> How do you implement something like the "abstract" keyword in Java ?
> Or: Is there a technique to force a subclass to implement a special
> method ?

One way is:

class Parent
   def abstract_method
     raise NotImplementedError
   end
end

You may just want to skip this step altogether though, since an
exception toss is the default behavior.

Hmm, I would say no, you will be caught by #method_missing one day.
It would be interesting why you want to do this?

For documentation purpose? ( Dumbs up !!)

Because you want the feature? (Dumbs down :wink:
I have not really yet seen a need for an abstract method in Ruby.
If you are looking for warnings or errors for subclasses not
implementing the abstract method you will be disappointed at Compile
Time [which arguabley does not even exist in 1.8]

If you are happy with the runtime exception (because you do nice code
covering tests :wink: James' solution is ideal, anyway you cannot do
better ( or worse for must of us;) in Ruby.

Welcome to the word of dynamic typing. :wink:

Yes indeed, welcome :slight_smile:

James Edward Gray II

Robert

···

On 7/12/07, James Edward Gray II <james@grayproductions.net> wrote:

On Jul 11, 2007, at 5:40 PM, cypher.dp@gmail.com wrote:

--
I always knew that one day Smalltalk would replace Java.
I just didn't know it would be called Ruby
-- Kent Beck

6

It's true that an exception toss is the default behavior, but it's a
NoMethodError instead of a NotImplementedError and that could be an
important distinction, depending. And I agree with Robert Dober that
it could get you caught by method_missing one day.

What gets me about NoMethodError is you get that whether there's no
method at all (which follows from the name) or as a result of going
against access control (calling a private or protected method).

···

On Jul 11, 5:44 pm, James Edward Gray II <j...@grayproductions.net> wrote:

On Jul 11, 2007, at 5:40 PM, cypher...@gmail.com wrote:

> How do you implement something like the "abstract" keyword in Java ?
> Or: Is there a technique to force a subclass to implement a special
> method ?

One way is:

class Parent
   def abstract_method
     raise NotImplementedError
   end
end

You may just want to skip this step altogether though, since an
exception toss is the default behavior.

Welcome to the word of dynamic typing. :wink:

James Edward Gray II

--
-yossef

7

> How do you implement something like the "abstract" keyword in Java ?
> Or: Is there a technique to force a subclass to implement a special
> method ?

One way is:

class Parent
   def abstract_method
     raise NotImplementedError
   end
end

You may just want to skip this step altogether though, since an
exception toss is the default behavior.

Hmm, I would say no, you will be caught by #method_missing one day.
It would be interesting why you want to do this?

If someone adds a method_missing() implementation to a child class that accepts messages it shouldn't, there is a bug in that implementation and it needs to be fixed. Your tests are going to catch that for you.

For documentation purpose?

That is about the best reason to add the stub, I think.

If you are looking for warnings or errors for subclasses not
implementing the abstract method you will be disappointed at Compile
Time [which arguabley does not even exist in 1.8]

class AbtractClass
   REQUIRED_METHODS = %w[one two three]

   def self.inherited(subclass)
     REQUIRED_METHODS.each do |m|
       raise "An implementation of #{m} is required by #{self.class}" \
         unless subclass.instance_methods.include? m
     end
   end
end

class Broken < AbtractClass
   def one; end
   def two; end
end
# ~> -:7:in `inherited': An implementation of one is required by Class (RuntimeError)
# ~> from -:5:in `each'
# ~> from -:5:in `inherited'
# ~> from -:12

But don't do that. :wink:

James Edward Gray II

···

On Jul 12, 2007, at 7:05 AM, Robert Dober wrote:

On 7/12/07, James Edward Gray II <james@grayproductions.net> wrote:

On Jul 11, 2007, at 5:40 PM, cypher.dp@gmail.com wrote:

8

Hi,

Not sure if this has been mentioned already, but you might also be interested in checking out http://rubyforge.org/projects/abstract/

George

···

On 12 Jul 2007, at 14:32, James Edward Gray II wrote:

On Jul 12, 2007, at 7:05 AM, Robert Dober wrote:

On 7/12/07, James Edward Gray II <james@grayproductions.net> wrote:

On Jul 11, 2007, at 5:40 PM, cypher.dp@gmail.com wrote:

> How do you implement something like the "abstract" keyword in Java ?
> Or: Is there a technique to force a subclass to implement a special
> method ?

One way is:

class Parent
   def abstract_method
     raise NotImplementedError
   end
end

You may just want to skip this step altogether though, since an
exception toss is the default behavior.

Hmm, I would say no, you will be caught by #method_missing one day.
It would be interesting why you want to do this?

If someone adds a method_missing() implementation to a child class that accepts messages it shouldn't, there is a bug in that implementation and it needs to be fixed. Your tests are going to catch that for you.

For documentation purpose?

That is about the best reason to add the stub, I think.

If you are looking for warnings or errors for subclasses not
implementing the abstract method you will be disappointed at Compile
Time [which arguabley does not even exist in 1.8]

class AbtractClass
  REQUIRED_METHODS = %w[one two three]

  def self.inherited(subclass)
    REQUIRED_METHODS.each do |m|
      raise "An implementation of #{m} is required by #{self.class}" \
        unless subclass.instance_methods.include? m
    end
  end
end

class Broken < AbtractClass
  def one; end
  def two; end
end
# ~> -:7:in `inherited': An implementation of one is required by Class (RuntimeError)
# ~> from -:5:in `each'
# ~> from -:5:in `inherited'
# ~> from -:12

But don't do that. :wink:

James Edward Gray II

9

<snip>

> If you are looking for warnings or errors for subclasses not
> implementing the abstract method you will be disappointed at Compile
> Time [which arguabley does not even exist in 1.8]

class AbtractClass
   REQUIRED_METHODS = %w[one two three]

   def self.inherited(subclass)
     REQUIRED_METHODS.each do |m|
       raise "An implementation of #{m} is required by #{self.class}" \
         unless subclass.instance_methods.include? m
     end
   end
end

class Broken < AbtractClass
   def one; end
   def two; end
end
# ~> -:7:in `inherited': An implementation of one is required by
Class (RuntimeError)
# ~> from -:5:in `each'
# ~> from -:5:in `inherited'
# ~> from -:12

But don't do that. :wink:

That does not work as you have intended, it is complaining about #one
missing as a matter of fact:

531/31 > cat subclass.rb && ruby subclass.rb
# vim: sw=2 ts=2 ft=ruby
class P
    def self.inherited(subclass)
                        puts "P --> #{subclass}"
    end
end

class C < P
                puts "in C"
end
P --> C
in C

Robert

···

On 7/12/07, James Edward Gray II <james@grayproductions.net> wrote:

--
I always knew that one day Smalltalk would replace Java.
I just didn't know it would be called Ruby
-- Kent Beck

10

You are right. My mistake.

James Edward Gray II

···

On Jul 12, 2007, at 9:17 AM, Robert Dober wrote:

On 7/12/07, James Edward Gray II <james@grayproductions.net> wrote:
<snip>

> If you are looking for warnings or errors for subclasses not
> implementing the abstract method you will be disappointed at Compile
> Time [which arguabley does not even exist in 1.8]

class AbtractClass
   REQUIRED_METHODS = %w[one two three]

   def self.inherited(subclass)
     REQUIRED_METHODS.each do |m|
       raise "An implementation of #{m} is required by #{self.class}" \
         unless subclass.instance_methods.include? m
     end
   end
end

class Broken < AbtractClass
   def one; end
   def two; end
end
# ~> -:7:in `inherited': An implementation of one is required by
Class (RuntimeError)
# ~> from -:5:in `each'
# ~> from -:5:in `inherited'
# ~> from -:12

But don't do that. :wink:

That does not work as you have intended, it is complaining about #one
missing as a matter of fact: