From: Kristof Bastiaensen <kristof@...>
Date: 2004-04-28T06:54:12+09:00
Subject: Re: Semantics of Multiple Values (updated RCR)

Hi,

On Wed, 28 Apr 2004 04:51:59 +0900, Jean-Hugues ROBERT wrote:
> 
> "unify" and "assign" work even better if a new type of
> value is introduced (in addition to the one introduced already,
> the one that is a & reference to a variable).
> 
> The new type is "FreeClass", much like TrueClass. That is
> a Singleton too.
> 
> Q. What is it for ?
> A. a = FreeClass.instance() is the way to "free" the "bound"
> variable named "a". Shorthand: a = free

This type already exists: NilClass, with as only
instance nil.  In fact nil is always used for unbound variables.
Whenever a variable isn't bound, it evaluates to nil, for all
variables (global, object) except local.  Local variables are
the only ones that can raise an error, the reason for this is
that local variables and methods or interchangeable.  Whenever
Ruby sees a local variable that didn't have an assignment before,
it treats it as a method call.  When at runtime it executes the
method, and cannot find it, it will give an error.

I would suggest using := as a unification operator.  Ruby could
then always see that what is before the operator are variables.
It would treat nil as the unbound.
I am still not really convinced of the practical use of this
operator.  I can see one usage, as initialization:

a := 0 #meaning a = 0 if (a == nil).

perhaps the following

def search_data
  some_operation
  return succeed, value1, value2
end

a, b = nil
if true, a, b := search_data
  <use a, b>
end

#but you may as well use
succes, a, b = search_data
if(succes)
  <use a, b>
end

There may be other real world practical uses, but I cannot think
of any.
Any suggestions?