Ruby: Boolean Challenge

Ruby
1

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[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

2

Errata:

Instead of:

puts (OddRecognizer.new.knows?(1) or OddRecognizer.new.knows?(3)) ?
“there are odds” : “there is no odd”

First example should be

recognizer = OddRecognizer.new
puts (recognizer.knows?(1) or recognizer.knows?(3)) ? “there are odds”
: “there is no odd”

That needs to be changed to:

puts OddRecognizer.new.knows?(1).or?(3) ? “there are odds” : “there is
no odd”

3

heh…
this will work… but i wouldn’t suggest using it for anything other
than intellectual enjoyment.

class FalseOddRecognizer < NilClass
//#same methods as TrueOddRecognizer
end

class TrueOddRecognizer
//#same methods as before
end

Now, in knows(), return a FalseOddRecognizer when false and a
TrueOddRecognizer on true.

The trick is that Ruby evaluates all subclasses of NilClass to false.

4

The trick is that Ruby evaluates all subclasses of NilClass to false.

The problem is that, if I extend NilClass, I wont be able to use new
anymore

class Test < NilClass; end;

Test.new # NoMethodError: undefined method `new’ for Test:Class

5

On 8/15/08, Emmanuel O. [email protected] wrote:

@recognizer.knows?(3).should be_false
end
end

Maybe I’m dense, but if the recognizer knows 2 but not 3 … doesn’t
that make it an EvenRecognizer?

I know this doesn’t effect the fundamental problem, but the examples
might be easier to understand if the names were consistent.

6

On Fri, Aug 15, 2008 at 1:04 PM, Emmanuel O.
[email protected] wrote:

–
Posted via http://www.ruby-forum.com/.

I’m not sure I understand what you’re looking for, but this passes your
specs.

Actually, I think there was a typo on line 94.

  • @recognizer.mknows?(1).or?(3).or?(5).should be_true
  • @recognizer.mknows?(1).or?(3).or?(6).should be_true

After I fixed that, it passes the specs.

7

Hi –

On Sat, 16 Aug 2008, Emmanuel O. wrote:

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

Do you mean EvenRecognizer?

David

8

Hi David

class OddRecognizer
def knows?(value)
(value % 2) == 0
end
end

Do you mean EvenRecognizer?

True, I confused the two terms (even, odd). I tend to make that mistake,
I also then to confuse the words Tuesday and Thursday :slight_smile:

9

Hey,
Sorry for the late reply: I was out to lunch:

I didn’t know you can’t instantiate NilClass.
Anyway here’s a workaround involving singletons:

class OddRecognizer
def knows(num)
return self if num%2==0

o = nil
class << o
  def or(num)
    return self if num%2==1
    return OddRecognizer.new
  end
end
return o

end

alias :or :knows
end

It was an interesting question from a intellectual standpoint.
But I wouldn’t touch this code with a ten foot pole. lol

-Patrick

10

class OddRecognizer
def knows(num)
return self if num%2==0
class << nil
def or(num)
return self if num%2==1
return OddRecognizer.new
end
end
nil
end

alias :or :knows
end

Patrick:

that sure works, the problem is that nil is extended with a or method
that is not very intuitive for it…

nil.or(2)
=> #OddRecognizer:0xb7a626bc what the ·$%!$%!$%@!!!

Code pollution at is maximum expression hahaha

I thought of doing something like that with false instead of nil, but
is the same problem. And you can’t dup nil true nor false obviously :slight_smile:

It was an interesting question from a intellectual standpoint.
But I wouldn’t touch this code with a ten foot pole. lol

Hahaha I agree.

11

Lol. Mmm i honestly didn’t see that one coming.
I guess nil and false are true singletons.
Perhaps there’s a hack out there that lets us instantialize a NilClass
object.

Anyway. Thanks for the question. That was interesting.

12

5619’s gists · GitHub

Actually, I think there was a typo on line 94.

  • @recognizer.mknows?(1).or?(3).or?(5).should be_true
  • @recognizer.mknows?(1).or?(3).or?(6).should be_true

After I fixed that, it passes the specs.

Yes I had a couple of typos, I think I have fixed them now. I’m not sure
about your change though, but I think the last edit I made makes more
clear what I’m needing:

it “should return false for 1 or 3 or 5 without having to call true at
the end” do
pending “Impossible in ruby?” do
lambda do
# needed functionality is to remove last true? and make it work
anyway.
raise if @recognizer.mknows?(1).or?(3).or?(5)#.true?
end.should_not raise_error
end
end

P.S.: GIST RULES!!! :smiley: