From: ptkwt@... (Phil Tomson)
Date: 2004-07-16T13:52:16+09:00
Subject: Re: Do you want a ":=" operator?

In article <9e7db91104071518595087c46@mail.gmail.com>,
Austin Ziegler  <halostatue@gmail.com> wrote:
>On Fri, 16 Jul 2004 10:47:15 +0900, Xiangyu  Yang
><xiangyu.yang@gmail.com> wrote:
>> gabriele renzi wrote:
>> > If you're working with hardware check out
>> > http://www.aracnet.com/~ptkwt/ruby_stuff/RHDL/
>> > Ruby Hardware Description Language :)
>> An interest thing.
>> 
>> In your RHDL you have to use .assign or .<< to do signal assignment,
>> annoying thing also in my code for HW CPU model.
>> 
>> In ruby, the "=" will assign other *object* to a *variable*, and can't
>> (and shouldn't) be overrided. There are no simple way to modify the
>> state of an *object* referenced by a *variable*. Using .assign or
>> overriding other operators to do this job are both ugly. I notice that
>> someone suggested ":=" operator which can be overrided. I dream of it
>> all the days :). If I have ":=", I'm sure that my CPU model will save
>> at least half of the current lines and the program will be much more
>> natural and clearer.
>> 
>> In short, when you use ruby to model some objects, the ability to
>> assign/modify the *object* state/value in simple expression way is very
>> useful. It provides the possibility of running ruby code *directly* to
>> model other worlds.
>
>Right. But not all object states or values can be changed that way,
>and there is no meaningful default operation for such a behaviour.
>Thus, it makes more sense to use the appropriate #<< or #assign
>mechanism.
>

The problem is that I might want to use #<< as 'shift left' in an HDL (or 
use it to append to a list as is it's normal semantics for lists).  IF we 
had a := operator it would be an optional operator that you could define 
for your class or not.  If we had :=  I suspect that the 'meaningful 
default operation' would evolve in this direction (lots of folks [newbies 
primarily] wonder about overriding '=' which is obviously bad, but a ':=' 
would allow them to mimic the desired behaviour).

Just to reiterate the proposed usage of ':=', here's an example based on 
the Bit class in RHDL:

class Bit #bits can take on the values 1,0,X,Z
  def initialize(value)
    assign(value)
  end

  def assign(value)
    #omitted code to make sure value is a valid value
    @value = value
  end
  
  def :=(value)
    assign(value)
  end
  #lost of other methods ommited
end

x = Bit.new(0)
....later...

x := 'Z' #the meaning seems quite clear 
#looks better than:
x.assign 'Z'

#and if I have a BitVector (another class which defines an array of Bit)
#I could then do:

bv = BitVector("1001")
bv << 1  #shift operator, shift left 1 position
....later...
bv := "ZZZZ"

So in the case of a BitVector, if I had a ':=' operator I would then be 
able to use '<<' to shift (it makes more sense) and not assignment.

I'd certainly like to see a := operator in upcoming versions of Ruby. The 
last time it came up, Matz didn't seem to be too opposed to the idea so 
maybe there's hope. ;-)

Phil