BigDecimal singletons


#1

Hi,

I’m a ruby nuby and I’m confused about something. When I try to create
a singleton method for a BigDecimal like so:

require ‘bigdecimal’
d = BigDecimal.new(‘10’)

class << d
def value
32
end
end

p.value

I get an error:
singtest.rb:7:in `singleton_method_added’: can’t define singleton method
“value” for BigDecimal (TypeError)

Doing this through IRB, I was surprised that if I then ran…

d.singleton_methods.sort

…after getting the error above, I get “value” listed as a singleton
method, and indeed, d.value yields 32.

If I wrap the singleton method definition in a begin…rescue block, the
TypeError is caught and not displayed, and the singleton method is
accessible.

If I replace d = BigDecimal.new(‘10’) with d = String.new(‘10’), but
leave the rest of the code the same, I don’t get the error and the
singleton method works as expected. Is there something simple I’m
missing?

Thanks in advance…

Dave


#2

On Sat, May 16, 2009 at 3:32 PM, Dave B. removed_email_address@domain.invalid
wrote:

32

leave the rest of the code the same, I don’t get the error and the
singleton method works as expected. Is there something simple I’m
missing?

The Numeric class overrides singleton_method_added to raise an error

$ ri -T Object#singleton_method_added
------------------------------------------ Object#singleton_method_added
singleton_method_added(symbol)

 From Ruby 1.8

 Invoked as a callback whenever a singleton method is added to the
 receiver.

    module Chatty
      def Chatty.singleton_method_added(id)
        puts "Adding #{id.id2name}"
      end
      def self.one()     end
      def two()          end
      def Chatty.three() end
    end

 _produces:_

    Adding singleton_method_added
    Adding one
    Adding three

$ ri -T Numeric#singleton_method_added
----------------------------------------- Numeric#singleton_method_added
singleton_method_added(p1)

 From Ruby 1.8

 Trap attempts to add methods to +Numeric+ objects. Always raises a
 +TypeError+

I guess this is to avoid surprises since certain kinds of numerics
can’t have singleton methods (e.g. Fixnums which because of their
implementation don’t have a klass pointer so can’t acquire a singleton
class). So it wouldn’t make sense to be able to add a singleton
method to 100 factorial (a Bignum), but not to 100 (a Fixnum).

However, the singleton_method_added callback happens after the method
has been added so it’s kind of like locking the barn door after the
horse has left.


Rick DeNatale

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