TrueClass/FalseClass vs. Boolean


#1

Greetings!

I am wondering why there are two separate classes (TrueClass and
FalseClass) to deal with boolean types insead of one class (something
like Boolean?). Do you know
why the library designer desided to use this approach?

Your help will be greatly apreciated.

Thanks


#2

On Mar 30, 2006, at 1:31 PM, PrimaryKey wrote:

Greetings!

I am wondering why there are two separate classes (TrueClass and
FalseClass) to deal with boolean types insead of one class (something
like Boolean?). Do you know
why the library designer desided to use this approach?

Your help will be greatly apreciated.

This has been discussed many, many times before. Here’s a reading list:

http://blade.nagaokaut.ac.jp/cgi-bin/vframe.rb?key=TrueClass
+FalseClass&cginame=namazu.rb&submit=Search&dbname=ruby-
talk&max=50&whence=0


Eric H. - removed_email_address@domain.invalid - http://blog.segment7.net
This implementation is HODEL-HASH-9600 compliant

http://trackmap.robotcoop.com


#3

Since everything in ruby is an object, there need to be objects for
representing true and false. Once you have those, why do you need a
Boolean class?


#4

PrimaryKey wrote:

I am wondering why there are two separate classes (TrueClass and
FalseClass) to deal with boolean types insead of one class (something
like Boolean?). Do you know
why the library designer desided to use this approach?

I think it is because true and false don’t really have anything in
common in Ruby. No way to share code.

You can always fix it pretty easily:

module Boolean; end
[true, false].each do |obj|
obj.extend(Boolean)
end

true.is_a?(Boolean) # => true
false.is_a?(Boolean) # => true

But note that in Ruby all objects (except false and nil) can be used as
true values. And they are used for just that even in the standard
library.


#5

On Mar 31, 2006, at 9:27 AM, Christian N. wrote:

case y
when Integer
when Boolean
end

case y
when Integer
when TrueClass,FalseClass
end

works ok, right? I kind of wish the class names were just True and
False.

Gary W.


#6

“removed_email_address@domain.invalid” removed_email_address@domain.invalid writes:

Since everything in ruby is an object, there need to be objects for
representing true and false. Once you have those, why do you need a
Boolean class?

if a.kind_of? Boolean

case y
when Integer
when Boolean
end

I often wanted this shortcut…


#7

On Fri, 31 Mar 2006, Christian N. wrote:

when Boolean
end

I often wanted this shortcut…

indeed. i tend to use

case obj
when Klass
when TrueClass, FalseClass
end

which is ugly.

another thing a real Boolean class could give is a ‘maybe’ obj such that

 maybe or true                   -> maybe
 maybe or true and false or true -> maybe
 maybe and false                 -> maybe

although i can’t think of anything attm to do with this it would
undoubtably
lead to some nice logical constructs that more closely parallel the way
we
think.

i’ve brought this up once or twice before - maybe we should just put
together
a patch and send to ruby-core?

regards.

-a


#8

removed_email_address@domain.invalid wrote:

On Fri, 31 Mar 2006, Christian N. wrote:

if a.kind_of? Boolean

case y
when Integer
when Boolean
end

What kind of scenarios would you want to use such a construct? It seems
to me not very rubyish to be switching based on the class of an object.
As I understand it, the more conventional way to deal with potentially
diverse argument types is to use the respond_to? method. This keeps the
door open for duck typing.

another thing a real Boolean class could give is a ‘maybe’ obj such that

although i can’t think of anything attm to do with this it would undoubtably
lead to some nice logical constructs that more closely parallel the way we
think.

It sounds like you’re talking about a sort of restricted fuzzy logic,
which is very cool, but is not appropriate for the core of a general
purpose language. Considering that every expression can be interpreted
in a boolean context, what would a maybe value do? Either behavior
seems wrong.


#9

“removed_email_address@domain.invalid” removed_email_address@domain.invalid writes:

to me not very rubyish to be switching based on the class of an object.
As I understand it, the more conventional way to deal with potentially
diverse argument types is to use the respond_to? method. This keeps the
door open for duck typing.

Except we don’t have a to_bool, either ;-).

Still, I can saw enough cases where I wanted such a “case”, and
grumbly used TrueClass,FalseClass.


#10

On Sat, 1 Apr 2006, removed_email_address@domain.invalid wrote:

me not very rubyish to be switching based on the class of an object. As I
understand it, the more conventional way to deal with potentially diverse
argument types is to use the respond_to? method. This keeps the door open
for duck typing.

i generally do use duck typing, but sometimes it is simply not
appropriate or
become too verbose. for example, consider a matrix class with the
following
behaviour

1d

 m[true]             #=> all data returned
 m[]                 #=> all data returned
 m[true] = elems     #=> data along all dims set
 m[]     = elems     #=> data along all dims set
 m[int]              #=> treat as 1d array, return elem
 m[int] = elem       #=> treat as 1d array, set idx to elem
 m[off, len]         #=> treat as 1d array, return elems off thru 

len
m[off, len] = elem #=> treat as 1d array, set elems off thru len
to elem
m[a … b] #=> treat as 1d array, return elems in range
m[a … b] = elem #=> treat as 1d array, set elems in range to
elem
m[n] #=> idx one matrix by another

2d

 generalize all above so multiple indices access in 

multi-dimensional
fashion. eg

   m[0,1,2]         #= return idx x,y,z
   m[0,30..42,true] #= return column 0, rows 30 to 42, in all height 

dims

now, in case you are thinking that i’m just being difficult, this is
exactly
how the narray class works

http://narray.rubyforge.org/SPEC.en

have fun writing that with respond_to? !! :wink:

also, when working with concrete data structures (like binary output
from C
programs) it’s often quite sufficient to have a type mapping. further
more
it’s sometimes critical, for instace writing the word located at
offset=42,
len=4 with a packed int value or sending a given sequence of bytes down
a
socket that a given type is used and it’s much more natural to write

def send buf, which = nil
case which
when Fixnum
socket.write [which].pack(‘N’)
socket.write buf
when NilClass
socket.write [buf.size].pack(‘N’)
socket.write buf
when Range
length = which.last - which.first
socket.write [length].pack(‘N’)
socket.write buf[which]
else
raise TypeError, which.class
end
end

than something using respond_to?.

don’t get me wrong - i defintely advocate duck typing, but when the
number of
possible input types becomes large and behaviour is different depending
on
that type it becomes cumbersome. this is the price we pay for not
having c–
style polymorphism. in fact, it’s very common to see the two styles
combined:

case arg
when Fixnum

when Range

else # dunno - assume String and use in a duck type fashion
end

context, what would a maybe value do? Either behavior seems wrong.
i disagree - ruby (and hardware) already supports this style of logic in
several ways:

harp:~ > irb
irb(main):001:0> nan = 0.0/0.0
=> NaN
irb(main):002:0> nan * 42
=> NaN
irb(main):003:0> nan * 42 + 42.0
=> NaN
irb(main):004:0> obj = Object.new and obj.taint
=> #Object:0xb75a5fac
irb(main):005:0> (obj.to_s << “string”).tainted?
=> true
irb(main):006:0> (“string” << obj.to_s).tainted?
=> true

there’s no good reason, imho, why this style of logical behaviour could
not be
part of the logical classes (TrueClass/FalseClass) and operators (and,
or,
not).

regards.

-a


#11

On Sat, Apr 01, 2006 at 12:38:43AM +0900, removed_email_address@domain.invalid wrote:
} removed_email_address@domain.invalid wrote:
} > On Fri, 31 Mar 2006, Christian N. wrote:
} > > if a.kind_of? Boolean
} > >
} > > case y
} > > when Integer
} > > when Boolean
} > > end
}
} What kind of scenarios would you want to use such a construct? It
seems
} to me not very rubyish to be switching based on the class of an
object.
} As I understand it, the more conventional way to deal with potentially
} diverse argument types is to use the respond_to? method. This keeps
the
} door open for duck typing.
[…]

The most obvious answer is that it is needed for various kinds of
serialization. I wrote something like this just the other day. I needed
to
do XML-RPC serialization (yes, I know there is XML-RPC support in the
standard library, but I needed something slightly different that was
easier
to do by hand), so I needed to know what kind of value I was
serializing.

–Greg


#12

unknown wrote:

 maybe or true                   -> maybe

Wouldn’t maybe or true be true?


– Jim W.


#13

removed_email_address@domain.invalid wrote:

end
You could always assign some new constants to them:

True = TrueClass
False = FalseClass

case y
when Integer
when True, False
end


#14

John W. Long wrote:

You could always assign some new constants to them:

True = TrueClass
False = FalseClass

case y
when Integer
when True, False
end

Or just use the literal constants, that already exist:

case y
when Integer
when true, false
end


#15

On Mar 31, 2006, at 10:21 AM, removed_email_address@domain.invalid wrote:

maybe and false -> maybe

Shouldn’t this be false?

{true, false} and false is going to be false regardless of what
maybe is


#16

On Mar 31, 2006, at 5:00 PM, Florian F. wrote:

Or just use the literal constants, that already exist:
case y
when Integer
when true, false
end

Another example where the Ruby ‘does the right thing’.
Thanks for the reminder.

Gary W.


#17

another thing a real Boolean class could give is a ‘maybe’ obj such that

maybe or true                   -> maybe
maybe or true and false or true -> maybe
maybe and false                 -> maybe

What whould this do

print "hello" if maybe

or this

while maybe
   print "hello"
end

// Niklas


#18

removed_email_address@domain.invalid wrote:

def maybe() [rand].pack(‘f’).unpack(‘c’).first[0].zero? end

What does

[rand].pack(‘f’).unpack(‘c’).first[0]

do that

rand(2)

doesn’t?


#19

On Sun, 2 Apr 2006, Joel VanderWerf wrote:

doesn’t?

sheesh - can’t you tell it looks longer!

:wink:

-a


#20

On Apr 1, 2006, at 10:03 AM, removed_email_address@domain.invalid wrote:

number and look at the last bit for instance, 0 -> false, 1 -> true.

  • h.h. the 14th dali lama

As if my code wasn’t already non-deterministic