Why does Ruby have TrueClass and FalseClass instead of a single Boolean class?
RubyLanguage DesignRuby Problem Overview
I was working on serializing values when found out about this one. Ruby has a TrueClass
class, and a FalseClass
class, but it has no Boolean
class. I'd like to know why is this.
I see some advantages in using a Boolean
; for example, string parsing could be centralized on it.
Ruby developers are smarter than me, so there must be a lot of good reasons that I just don't see. But right now it looks to me like having OneClass
and a TwoClass
instead of Fixnum
.
Ruby Solutions
Solution 1 - Ruby
The purpose of a class is to group similar objects, or objects with similar behavior together. 1
and 2
are very similar, therefore it makes perfect sense for them to be in the same class. true
and false
however are not similar. In fact, their whole point is that they are exactly the opposite of each other and have opposite behavior. Therefore, they don't belong in the same class.
Can you give an example of what sort of common behavior you would implement in a Boolean
class? I can't think of anything.
Let's just look at the behavior that TrueClass
and FalseClass
have: there's exactly four methods there. No more. And in every single case, the two methods do exactly the opposite. How and why would you put that in a single class?
Here's how you implement all those methods:
class TrueClass
def &(other)
other
end
def |(_)
self
end
def ^(other)
!other
end
def to_s
'true'
end
end
And now the other way around:
class FalseClass
def &(_)
self
end
def |(other)
other
end
def ^(other)
other
end
def to_s
'false'
end
end
Granted, in Ruby, there is a lot of "magic" that is going on behind the scenes and that is not actually handled by TrueClass
and FalseClass
but rather hardwired into the interpreter. Stuff like if
, &&
, ||
and !
. However, in Smalltalk, from which Ruby borrowed a lot including the concept of FalseClass
and TrueClass
, all of these are implemented as methods as well, and you can do the same thing in Ruby:
class TrueClass
def if
yield
end
def ifelse(then_branch=->{}, _=nil)
then_branch.()
end
def unless
end
def unlesselse(_=nil, else_branch=->{})
ifelse(else_branch, _)
end
def and
yield
end
def or
self
end
def not
false
end
end
And again the other way around:
class FalseClass
def if
end
def ifelse(_=nil, else_branch=->{})
else_branch.()
end
def unless
yield
end
def unlesselse(unless_branch=->{}, _=nil)
ifelse(_, unless_branch)
end
def and
self
end
def or
yield
end
def not
true
end
end
A couple of years ago, I wrote the above just for fun and [even published it][1]. Apart from the fact that the syntax looks different because Ruby uses special operators while I use only methods, it behaves exactly like Ruby's builtin operators. In fact, I actually took [the RubySpec conformance testsuite][2] and [ported it over to my syntax][3] and it passes. [1]: http://JoergWMittag.GitHub.Com/b001e/ [2]: https://GitHub.Com/RubySpec/RubySpec/blob/master/language/if_spec.rb [3]: https://GitHub.Com/JoergWMittag/B001e/tree/master/spec/
Solution 2 - Ruby
It seems that Matz himself answered this question on a mailing list message in 2004.
Short version of his answer: "right now it works ok, adding a Boolean doesn't give any advantage".
Personally I don't agree with that; the aforementioned "string parsing" is one example. Another one is that when you are applying different treatment to a variable depending on its type, (i.e. a yml parser) having a "Boolean" class is handy - it removes one "if". It also looks more correct, but that's a personal opinion.
Solution 3 - Ruby
Quoting Matz on Ruby forum (2013):
> ...There's nothing true and false commonly share, thus no Boolean class. Besides that, in Ruby, everything behave as Boolean value....
Solution 4 - Ruby
true and false could be managed by a Boolean class that held multiple values, but then the class object would have to have internal values, and therefore have to be de-referenced with every use.
Instead, Ruby treats true and false as long values (0 and 1), each of which corresponds to a type of object class (FalseClass and TrueClass). By using two classes instead of a single Boolean class, each class does not require any values and therefore can be distinguished simply by its class identifier (0 or 1). I believe this translates to significant speed advantages internal to the Ruby engine, because internally Ruby can treat TrueClass and FalseClass as long values that require zero translation from their ID value, whereas a Boolean object would have to be de-referenced before it could be evaluated.
Solution 5 - Ruby
Since everything but false
and nil
evaluate to true in Ruby by default, you would only need to add parsing to String.
Something like this could work:
class Object
## Makes sure any other object that evaluates to false will work as intended,
## and returns just an actual boolean (like it would in any context that expect a boolean value).
def trueish?; !!self; end
end
class String
## Parses certain strings as true; everything else as false.
def trueish?
# check if it's a literal "true" string
return true if self.strip.downcase == 'true'
# check if the string contains a numerical zero
[:Integer, :Float, :Rational, :Complex].each do |t|
begin
converted_number = Kernel.send(t, self)
return false if converted_number == 0
rescue ArgumentError
# raises if the string could not be converted, in which case we'll continue on
end
end
return false
end
end
When used, this would give you:
puts false.trueish? # => false
puts true.trueish? # => true
puts 'false'.trueish? # => false
puts 'true'.trueish? # => true
puts '0'.trueish? # => false
puts '1'.trueish? # => true
puts '0.0'.trueish? # => false
puts '1.0'.trueish? # => true
I believe part of the “big idea” behind Ruby is to just make the behavior you desire inherent to your program (e.g. boolean parsing), rather creating a fully encapsulated class that lives in it's own namespace world (e.g. BooleanParser).
Solution 6 - Ruby
The main reason is simply the fact that this is far quicker and simpler to implement boolean expressions as it is currently than with a Boolean class which would imply a conversion.
As Mongus Pong told you, when you write "if
Remember that such a
Do you realize that it would replace a few CPU instructions operation by a complete method call, which is costly in Ruby (remember the "send" method processing)...
Solution 7 - Ruby
In Ruby nil and false are false and everything else is true. Hence there is no need for a specific boolean class.
You can try it :
if 5
puts "5 is true"
end
5 evaluates to true
if nil
puts "nil is true"
else
puts "nil is false"
end
Will print "nil is false"
Solution 8 - Ruby
As others have said, you could "patch" Ruby. Create your own class. Here is something I came up with. The methods on the Boolean class are a bit silly, but they could be useful programmatically at some point.
class Boolean
def self.new(bool)
bool
end
def self.true
true
end
def self.false
false
end
end
class FalseClass
def is_a?(other)
other == Boolean || super
end
def self.===(other)
other == Boolean || super
end
end
class TrueClass
def is_a?(other)
other == Boolean || super
end
def self.===(other)
other == Boolean || super
end
end