From: Florian Gross <flgr@...>
Date: 2004-12-01T08:12:46+09:00
Subject: Re: singleton methods : when are they not permitted?

itsme213 wrote:

> Why no singleton methods on Symbol?
> 
> Are there other objects that are also not allowed to have singleton methods?

Singleton methods can not be defined for "immediate" Objects. Immediate 
Objects are stored directly by their id which means they are very cheap 
to use.

Immediate objects include all Fixnums, Symbols, true, false and nil (but 
note that  def true.foo; end  will silently add the method directly to 
TrueClass because there is guaranteed to be only one instance, true, for 
it.)

You can also not add methods to Floats and Bignums (this is for 
consistency with Fixnums), but this can be changed by overriding 
Numeric#singleton_method_added.

It also seems not to be possible to directly add methods to literals 
like this: (Not sure why -- maybe Ruby optimizes literals?)

irb(main):014:0> def ("x").foo; end
SyntaxError: compile error
(irb):14: can't define singleton method for literals

However this can be worked around trivially by using an expression instead:

irb(main):015:0> def (("x")).foo; end
=> nil