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?
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.