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comp.lang.ruby

Truth value evaluating of an object

Dave River

10/27/2007 2:36:00 PM

I know ruby treat an object as false whenever it is nil or false.
However, I wonder if there are any other ways to change this behavior.

For example, I define a class called AreYouOk.
class AreYouOk
def initialize(ok)
@ok = ok
end
end

x = AreYouOk.new(false)
puts "you are ok" if x

Since x is not nil, ruby prints " you are ok".
However, I want ruby to make the decision based on the @ok instance
variable. Are there any ways to do that?

I know that there is a method called __bool__ in Python. You can define
your __bool__ method in your class. The truth value of an object is
based on the return value of __bool__. Does ruby provide similar
mechanism?
--
Posted via http://www.ruby-....
5 Answers

Arlen Cuss

10/27/2007 2:50:00 PM

0

Hi,

On Sat, 2007-10-27 at 23:35 +0900, Dave River wrote:
> I know that there is a method called __bool__ in Python. You can define
> your __bool__ method in your class. The truth value of an object is
> based on the return value of __bool__. Does ruby provide similar
> mechanism?

I don't think I'm speaking out of school when I say that there's no such
way to do so.

Ruby and libraries likely rely on 'if obj' returning false if it's false
or nil - overriding this behaviour will probably cause some strange
behaviour which may result in your object being inadvertently modified!

Instead, just make an accessor like ok?, so you can test "if x.ok?".

If you need some way to test this across several objects, consider this:

class Object
def ok?
self
end
end
class AreYouOk
def ok?
@ok
end
end

Thus 'if x.ok?' returns the same as 'if x' for any object, but @ok for
yours!

Cheers
Arlen

Dave River

10/27/2007 3:19:00 PM

0


> Thus 'if x.ok?' returns the same as 'if x' for any object, but @ok for
> yours!


Thanks for your explanation!

In fact, I am making a wrapper class called Boolean which cooperates
with some legacy code in my company because there are some compatibility
problems between different languages.

I write some code like the following and the Boolean object hides some
underlying code which solve the compatibility problems.
x = Boolean.new()
if x
do something....
end

If ruby does not support something like __bool__ in Python, I need to
write some code in the following way.
x = Boolean.new()
if x.evaluate()
do something...
end

But I would prefer "if x " instead of "if x.evaluate" because it is more
straight forward. So, I would like to know whether there are any ways to
do so.

Regards,
Dave
--
Posted via http://www.ruby-....

Phrogz

10/27/2007 3:38:00 PM

0

On Oct 27, 9:18 am, Dave River <carlos_la...@yahoo.com.hk> wrote:
> I write some code like the following and the Boolean object hides some
> underlying code which solve the compatibility problems.
> x = Boolean.new()
> if x
> do something....
> end
>
> If ruby does not support something like __bool__ in Python, I need to
> write some code in the following way.
> x = Boolean.new()
> if x.evaluate()
> do something...
> end
>
> But I would prefer "if x " instead of "if x.evaluate" because it is more
> straight forward. So, I would like to know whether there are any ways to
> do so.

There are not any ways to do so in Ruby. Sorry. (This question comes
up every few weeks or so.)

Robert Klemme

10/27/2007 4:19:00 PM

0

On 27.10.2007 17:18, Dave River wrote:
>> Thus 'if x.ok?' returns the same as 'if x' for any object, but @ok for
>> yours!
>
>
> Thanks for your explanation!
>
> In fact, I am making a wrapper class called Boolean which cooperates
> with some legacy code in my company because there are some compatibility
> problems between different languages.
>
> I write some code like the following and the Boolean object hides some
> underlying code which solve the compatibility problems.
> x = Boolean.new()
> if x
> do something....
> end
>
> If ruby does not support something like __bool__ in Python, I need to
> write some code in the following way.
> x = Boolean.new()
> if x.evaluate()
> do something...
> end
>
> But I would prefer "if x " instead of "if x.evaluate" because it is more
> straight forward. So, I would like to know whether there are any ways to
> do so.

What exactly does Boolean do? Maybe you can get rid of it or do some
other changes so you can directly work with "true" and "false.

Kind regards

robert

Morton Goldberg

10/27/2007 11:31:00 PM

0

On Oct 27, 2007, at 10:35 AM, Dave River wrote:

> I know ruby treat an object as false whenever it is nil or false.
> However, I wonder if there are any other ways to change this behavior.
>
> For example, I define a class called AreYouOk.
> class AreYouOk
> def initialize(ok)
> @ok = ok
> end
> end
>
> x = AreYouOk.new(false)
> puts "you are ok" if x
>
> Since x is not nil, ruby prints " you are ok".
> However, I want ruby to make the decision based on the @ok instance
> variable. Are there any ways to do that?
>
> I know that there is a method called __bool__ in Python. You can
> define
> your __bool__ method in your class. The truth value of an object is
> based on the return value of __bool__. Does ruby provide similar
> mechanism?

What you are asking about looks to me like a flag class.

<code>
class Flag
def initialize(state=false)
@state = state
end
def set?
@state
end
def set
@state = true
end
def clear
@state = false
end
end

ok = Flag.new
puts "you are ok" if ok.set?
puts "you are not ok" unless ok.set?
ok.set
puts "you are ok" if ok.set?
puts "you are not ok" unless ok.set?

</code>

First the string "you are not ok" is printed. After the flag is set,
the string "you are ok" is printed.

Regards, Morton