From: Bill Atkins <batkins57@...>
Date: 2005-05-09T09:53:16+09:00
Subject: Re: No Thing Here vs Uninitialized and RCR 303

But Ruby already makes a distinction between nil and uninitialized.  Compare:

    b + 4
    NameError: undefined local variable or method `b' for main:Object

with

    b = nil; b + 4
    NoMethodError: undefined method `+' for nil:NilClass

In my example, file is certainly initialized - it's just initialized
to nil.  If create_new_log_file returns nil, file can't be considered
uninitialized.

On 5/8/05, John Carter <john.carter@tait.co.nz> wrote:
> On Mon, 9 May 2005, Bill Atkins wrote:
> 
> > This isn't about changing programming habits.  Having nil return nil
> > for missing methods can have serious consequences.
> >
> > Consider:
> >
> >   file = create_new_log_file
> >   file.log "here's a bit of information"
> >
> > You're expecting this code to log something to a file every time it
> > gets called.  But suppose create_new_log_file returns nil, due to some
> > mistake you've made while coding it.
> 
> You confirm what I am saying about the difference between "uninitialized
> and "no thing here". There should be an explicit difference.
> 
> Such a mistake you have presented would typically fill "file" with
> "uninitialized", and invoking the log method on "uninitialized" should
> throw an exception.
> 
> On the other hand, a nifty way of deliberately switching off logging,
> without changing a line of the client code would be to stuff "No thing
> here" into file, resulting in no thing happening.
> 
> That is why I have now withdrawn my RCR, and are working instead on an
> implementation of the Nothing class, leaving "nil" to mean
> "uninitialized".
> 
> It's coming along very nicely thank you, the only sorrow I have at the
> moment is to be able to do what I used to with nil takes a bit more
> typing....
> 
>    may_be_nothing = nil
> 
>    if may_be_nothing
>      definitely_isnt_nothing( may_be_nothing)
>    end
> 
> now becomes...
> 
>    may_be_nothing = Nothing.new # I love that
>    unless may_be_nothing.nothing?
>      definitely_isnt_nothing( may_be_nothing)
>    end
> 
> ie. "nil" is a Special object in ruby, and the code for handling "if"
> statements knows about it. (The RTEST macro in ruby.h)
> 
> But hopefully the change conditional to virtual method refactoring will
> make most of those instances go away.
> 
> John Carter                             Phone : (64)(3) 358 6639
> Tait Electronics                        Fax   : (64)(3) 359 4632
> PO Box 1645 Christchurch                Email : john.carter@tait.co.nz
> New Zealand
> 
> Surprisingly enough, this email does not originate from a beetle.
> 
> 


-- 
Bill Atkins