Forum: Ruby BigDecimal singletons

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C31cc64659c8c1bba84afc892eaba992?d=identicon&s=25 Dave Birch (daveb1976)
on 2009-05-16 21:32
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
8f6f95c4bd64d5f10dfddfdcd03c19d6?d=identicon&s=25 Rick Denatale (rdenatale)
on 2009-05-16 22:52
(Received via mailing list)
On Sat, May 16, 2009 at 3:32 PM, Dave Birch <thisisdaveb@gmail.com>
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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