From: Rick DeNatale <rick.denatale@...>
Date: 2007-06-08T04:15:26+09:00
Subject: Re: Globals not incrementing inside block

On 6/7/07, Todd A. Jacobs <tjacobs-sndr-019fdb@codegnome.org> wrote:
> On Thu, Jun 07, 2007 at 01:55:23PM +0900, Ivan Salazar wrote:
>
> > This one is a very common mistake when someone is new to functional
> > languages specially when having some procedural language experience...
> > Many methods in functional languages don't have side effects (don't
>
> Thanks. In retrospect, this is somewhat obvious. But you're right: it
> was my expectation that foo.succ was equivalent to foo+=1, rather than
> simply being an expression that returned a value without modifying the
> variable itself.

May I have the temerity to point out that there's another subtle
misconception in this statement which I see a lot of Ruby nubies trip
up on, and that's thinking that variables are objects.

An object holds state and behavior, a variable references an object.
More than one variable can reference the same object.

Modifying an object means modifying the objects state.

Modifying a variable means changing WHICH object it is referencing.
Which is done by assignment or assignment-like things such as
providing a value to the parameter of a method or block.

I would argue that: a = a.chop! doesn't change the variable a since
it's still referring to the same object.  Consider:

     a = "abc"
    # changes (sets) VARIABLE a

     b = a
     # changes (sets) VARIABLE b

     p a.object_id  => -606250168
     p b.object_id  => -606250168
     # note that its one OBJECT referenced by two VARIABLES

     c = a.delete('a')  => "bc"
     # creates a new OBJECT leaving both VARIABLEs
     # and the OBJECT they reference unchanged.
     p a => "abc"
     p b => "abc"
     p a.object_id  => -606250168
     p b.object_id  => -606250168

     b.delete!('b')
     # changes the state of the OBJECT referenced by VARIABLE b
     p b => "ac"

     p b.object_id  => -606250168
    # but leaves the VARIABLE b unchanged, it's still referencing the same
    # object albeit that object's state has changed.
     p a => "ac"
     p a.object_id  => -606250168
    # and VARIABLE a still refers to the same (changed) OBJECT

     b = c
     # changes the VARIABLE b
     p b.object_id => -606269188
     p c.object_id => -606269188


And variable is a general term covering (local|global|instance|class)
variables as well as things like the slots in Arrays which refer to
the elements of the array.


-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/