From: Emmanuel Oga <oga_emmanuel_oga@...>
Date: 2008-08-16T02:27:58+09:00
Subject: Ruby: Boolean Challenge

# Paste url: http://gist.github.com/5603
# Gist Paste Clone URL: git://gist.github.com/5603.git

# [Impossible?] CHALLENGE: See pending spec at the bottom
#
# PROBLEM:   I want to replace statements like this:
#
#   puts (OddRecognizer.new.knows?(1) or OddRecognizer.new.knows?(3)) ?
"there are odds" : "there is no odd"
#
# With something like this:
#   puts OddRecognizer.new.knows?(1).or?(3) ? "there are odds" : "there
is no odd"
#
# OddRecognizer is just a trivial class just to prove my concept
class OddRecognizer
  def knows?(value)
    (value % 2) == 0
  end
end

require 'rubygems'
require 'spec'

describe OddRecognizer do
  before :each do
    @recognizer = OddRecognizer.new
  end

  it "should recognize 2" do
    @recognizer.knows?(2).should be_true
  end

  it "should not recognize 3" do
    @recognizer.knows?(3).should be_false
  end
end

# Here's the implementation
class BooleanEvaluator

  Operation = Struct.new :name, :arguments

  def initialize(target, method, *args)
    @target, @method = target, method
    @arguments = Array.new << Operation.new(:first, args)
  end

  def or?(*args)
    @arguments << Operation.new(:or, args)
    self
  end

  def and?(*args)
    @arguments << Operation.new(:and, args)
    self
  end

  def true?
    @arguments.inject(false) do |result, operation|
      case operation.name
      when :or    then result or  @target.send(@method,
*operation.arguments)
      when :and   then result and @target.send(@method,
*operation.arguments)
      else                        @target.send(@method,
*operation.arguments)
      end
    end
  end
end

# For now let's add a new method that will behave like I want.
class OddRecognizer
  def mknows?(*args)
    BooleanEvaluator.new(self, :knows?, *args)
  end
end

# Let's test it.
describe OddRecognizer do
  before :each do
    @recognizer = OddRecognizer.new
  end

  it "should return false for 1 or 3 or 5" do
    @recognizer.mknows?(1).or?(3).or?(5).true?.should be_false
  end

  it "should return true for 1 or 3 or 6" do
    @recognizer.mknows?(1).or?(3).or?(6).true?.should be_true
  end

  # Here lies the problem: in ruby, everything that is not nil nor false
evaluates
  # as true in conditional expressions.... Is there any way to modify
BooleanEvaluator
  # (or implement it in a different way) to make this work? I don't
think so,
  # but maybe there is a way to implement something more close to the
thing I want.
  it "should return true for 1 or 3 or 6 without having to call true at
the end" do
    pending "Impossible in ruby?" do
      @recognizer.mknows?(1).or?(3).or?(5).should be_true
    end
  end
end
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