On Sat, May 16, 2009 at 3:32 PM, Dave Birch <thisisdaveb / gmail.com> wrote:
> Hi,
>
> I'm a ruby nuby and I'm confused about something.   
> a singleton method for a BigDecimal like so:
>
> require 'bigdecimal'
> d = BigDecimal.new('10')
>
> class << d
>   > 2
>  
> 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.   ɧ> 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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