From: "Gerardo Santana Gómez Garrido" <gerardo.santana@...>
Date: 2008-05-04T03:50:59+09:00
Subject: Re: Why there is not "replace" method for Fixnum?

On Sat, May 3, 2008 at 1:41 PM, Iñaki Baz Castillo <ibc@aliax.net> wrote:
> El Sábado, 3 de Mayo de 2008, Rick DeNatale escribió:
>
>
>
>  > The short answer is NO!, let's look more carefully at what's happening
>  > to see why.
>  >
>  > a = "original object value"
>  > # This binds the variable a to a string object.  The identity of the
>  > # string object can be obtained using the object_id method
>  >
>  > a.object_id                  # => 63930
>  >
>  > b = a
>  >
>  > # This binds b to the identical object to which a is currently bound so:
>  >
>  > b.object_id                  # => 63930
>  >
>  > b = "new value"
>  > # We've now bound b to a different object.
>  > b.object_id                  # => 63360
>  > b                            # => "new value"
>  >
>  > # But a is still bound to the first object
>  > a.object_id                  # => 63930
>  >
>  > # Now we re-bind b to the original object
>  > b = a
>  > b.object_id                  # => 63930
>  > b                            # => "original object value"
>  >
>  > # And send replace to that object.
>  > b.replace("new value")
>  > b.object_id                  # => 63930
>  > b                            # => "new value"
>  > a.object_id                  # => 63930
>  > a                            # => "new value"
>  >
>  > # Two points.  First, methods (like object_id, and replace) operate on
>  > objects, NOT variables.
>  > # Second, the replace method causes the string to change its contents,
>  > NOT its identity.  Some Ruby
>  > # objects are mutable, they have methods which can alter their state,
>  > other objects are immutable, once
>  > # created they can't be changed.
>  >
>  > a = "a new value"
>  > b = "a new value"
>  > a.object_id                  # => 60490
>  > b.object_id                  # => 60420
>  >
>  > # This illustrates that two strings with the same contents, aren't
>  > necessarily the same object.
>  > a == b                       # => true
>  > a.equal?(b)                  # => false
>  >
>  > # The == method compares state, equal? compares identity.
>  >
>  > # Some objects, such as Fixnums, Symbols, nil, true, and false have
>  > only one instance for a
>  > # particular state.
>  >
>  > a = 1
>  > b = 1
>  > a.object_id                  # => 3
>  > b.object_id                  # => 3
>  >
>  > Such objects certainly need to be immutable.  If there's only one
>  > instance of the fixnum 42, then you really wouldn't want to change its
>  > value to say 43, or life, the universe, and everything would lose
>  > their meaning.
>
>
>  I hope explanations like this would appear in some Ruby manual and so :)
>

What about the Fixnum class documentation:

http://www.ruby-doc.org/core/classes/Fixnum.html

"Fixnum objects have immediate value. This means that when they are
assigned or passed as parameters, the actual object is passed, rather
than a reference to that object. Assignment does not alias Fixnum
objects. There is effectively only one Fixnum object instance for any
given integer value, so, for example, you cannot add a singleton
method to a Fixnum."

-- 
Gerardo Santana