From: timr <timrandg@...>
Date: 2011-01-03T13:35:26+09:00
Subject: Re: 1.respond_to? :define_singleton_method # => true

It looks like there are two classes, Fixnum and Symbol, that produce
objects which cannot have singleton methods. This is the only
practical difference between these classes and regularly referenced
classes.

From Matz' Orielly book:

3.8.1.1 Immediate values
We've said that all values in Ruby are objects and all objects are
manipulated by reference. In the reference implementation, however,
Fixnum and Symbol objects are actually "immediate values" rather than
references. Neither of these classes have mutator methods, so Fixnum
and Symbol objects are immutable, which means there is really no way
to tell that they are manipulated by value rather than by reference.

The existence of immediate values should be considered an
implementation detail. The only practical difference between immediate
values and reference values is that immediate values cannot have
singleton methods defined on them. (Singleton methods are explained in
6.1.4)

Tim