From: Mark Hubbart <discord@...>
Date: 2004-04-24T13:27:06+09:00
Subject: Re: Creating bang methods


On Apr 23, 2004, at 2:37 PM, Jean-Hugues ROBERT wrote:

> At 05:52 24/04/2004 +0900, Jim Weirich wrote:
>
>> Jean-Hugues ROBERT said:
>> > About ++ ...
>> > At 03:48 24/04/2004 +0900, you wrote:
>> >>Just like +=, ++ can not be defined as a method on an object.
>> >
>> > but + can be redefined and ++ could as well.
>>
>> + is a method.  It is sent to an object.  (e.g.  1+2  ===  1.+(2) )
>>
>> = is not a method.  It is never sent to an object.  It binds objects 
>> to
>> names within a scope.  (e.g.   x=1  means that the name "x" is now 
>> bound
>> to the value "1")
>>
>> ++ cannot be a method because it changes bindings, just like =.
> Let me rephrase.
> If ruby was to understand:
>   this++
>   ++that
> as (syntax sugar for):
>   (tmp = this, this = something.postplusplus(), tmp)
>   (that = that.preplusplus())
> then one could redefine methods postplusplus() and preplusplus()
> (whose default definitions should be: self.plusplus(); in a decent 
> world).
>
> I understand that = is not a method as of today. And need not to
> be for ++ to be re definable (as rephrased).

IMHO, *if* it was decided to add ++ and -- to the language, it should 
be made obvious that ++ is not a method. Instead of translating var++ 
to var += 1, or defining a #++() method, it should replace the contents 
of var with var.succ, or var.pred. It might translate something like 
this:

   print var++
   print var--
   print ++var
   print --var

becomes the equivalent of:

   print lambda{|tmp|tmp=var;var=var.succ;tmp}[nil]
   print lambda{|tmp|tmp=var;var=var.pred;tmp}[nil]
   print (var = var.succ)
   print (var = var.pred)

Then all that would be needed would be to implement #succ and #pred for 
any object, and you can ++ and -- your heart out.

It should be pointed out out that var++ is *not* equivalent to var += 
1, as someone suggested. var++ returns the original value of var, then 
sets it to the incremented value.

> BTW: Making = a method (of class Binding I guess) would open
> interesting perspectives... ;-)

!!! I *really* hope that was a joke! :)

cheers,
--Mark