final_bob.rb

class Bob
  def reply_to(statement)
    public_send("reply_to_#{statement.class}".downcase.to_sym)
  rescue NoMethodError
    default_reply
  end

  def reply_to_silence
    "Fine. Be that way!"
  end

  def reply_to_yelling
    "Woah, chill out!"
  end

  def reply_to_question
    "Sure."
  end

  def default_reply
    "Whatever."
  end
end

class Statements
  def self.all
    [Question, Yelling, Silence, NullStatement]
  end
end

Statement = Struct.new(:statement)

class Question < Statement
  def self.match?(statement)
    statement.end_with?("?")
  end
end

class Yelling < Statement
  def self.match?(statement)
    statement.upcase == statement && statement.downcase != statement
  end
end

class Silence < Statement
  def self.match?(statement)
    statement.strip.empty?
  end
end

class NullStatement < Statement
  def self.match?(statement)
    true
  end
end

class StatementFactory
  def self.build(statement)
    Statements.all.detect { |s| s.match?(statement) }.new(statement)
  end
end

@amcaplan I agree with you regarding #2 — I don't love the use of exceptions for flow control. However I think this is a contrived example with the stated goal of zero conditionals, so maybe we shouldn't judge too harshly.

However regarding your gripe in #1:

We've basically now forced Bob to know the names of lots of other classes.

I don't see how Bob having a REPLIES hash with keys matching the names of Statement classes is any different from Bob having methods matching those names.

In addition, I don't love the Factory, nor the need to maintain an array of Statements in a separate class. Now if a programmer creates a new kind of Statement, she also needs to know about putting it into an array in a separate class. @mereghost's solution leaving instantiation to the superclass and using inherited? to keep track of all descendants seems the ideal solution to me.

@mereghost's answer is pretty interesting but I think it has a typo?

I think it should be like this:

self.inherited(klass)

No question mark in the name.

Ref: https://apidock.com/ruby/Class/inherited

@rmcsharry Yep. >.<