From: Matt Margolis <matt@...>
Date: 2007-10-10T01:07:35+09:00
Subject: Re: Make a class evaluate to false in boolean expressions

Logan Capaldo wrote:
> On 10/9/07, Matt Margolis <matt@mattmargolis.net> wrote:
>   
>> I want to have a class that always evaluates to false in boolean
>> expressions.  Is there a way to extend Ruby so that it evaluates an
>> object of a class to false like false and nil?
>>
>> Example
>>
>> class AlwaysFalse
>> end
>>
>> should_be_false = AlwaysFalse.new
>>
>> if should_be_false
>>   #I don't want this to ever run
>> else
>>   #will always run
>> end
>>
>> The use case for this is to have the concept of false but have it
>> contain instance variables.  You can't stick instance variables on
>> FalseClass because it is a singleton and every use of false will
>> overwrite the previous instance variable values.
>> a = false
>> a.name = "fish"
>> b = false
>> b.name = "ocean"
>> #at this point in time a.name is equal to "ocean" and not "fish" like I
>> want.
>>
>> This example may seem like an awful design decision but in the program I
>> am writing it is important to have an interface like this to meet the
>> requirements I have to code against.
>>
>> Is achieving this behavior as simple as redefining some operator methods
>> like == or is this sort of behavior not possible in Ruby?
>>
>>     
> The latter.
>
> OTOH:
>
> class Object
>   def false?
>     false
>   end
> end
>
> class FalseClass
>   def false?
>      true
>   end
> end
>
> class YourClass
>    def false?
>       true
>    end
> end
>
> if condition.false?
>  ...
> end
>
> Kind of annoying if you forget the .false? though. A better thing to
> do would be to figure out what you are trying to model by having
> multiple "falsy" values with some state.
>
>   
>> Thank you,
>> Matt Margolis
>>
>>
>>     
>
>   
I don't want to have to call .false? though.  Ideally what I would like 
to be able to do is just use the object in boolean expressions like 
false.  Anyone have any idea if this is at all possible?

Matt Margolis