From: Kristof Bastiaensen <kristof@...>
Date: 2004-04-27T09:19:03+09:00
Subject: Re: Semantics of Multiple Values

Hi,

On Tue, 27 Apr 2004 03:00:23 +0900, Jean-Hugues ROBERT wrote:
> Multiple Values, Assignments and *Unifications*
> 
> This RCR is really nice.
> The multiple assignment reminds me of Prolog's "unification" mechanism.
> What about some further generalization ?
> 
> Why not a generalized "assign" operator ? Syntax:
>    assign term1, term2 [, term3 [, ...]]
> And of course an "unify" operator, Syntax:
>    unify term1, term2 [, term3 [, ...]]
> 
> Result
>    def mymethod(); return 1, [2], 3 end # Returns an Array
>    a, b, c = mymethod         # => 3, a == 1, b == [2], c == 3
>    assign [a,[b]], mymethod   # => 2, a == 1, b == 2, c unchanged
>    assign [a,[],c], mymethod  # => false, [] instead of [2]
>    p unify [a,[2],3], mymethod  # => true
>    p unify r, mymethod        # => [1,[2],3]
>    p unify [a,_,_], mymethod  # => 1, _ means "ignore me"
> 

This looks very interesting, I hadn't thought so far.
(I also don't know so much about Prolog).  From the
point of view of syntax, it would be hard to implement,
because assign [a, [b]], mymethod is already valid syntax,
and would have to make assign and unify a special construct
or operator.  This may be not so clear.  I cannot say much
more about it since I am not into prolog.

> A key benefit of this proposal is that it does not change
> the current semantic of returned values. As a result it is
> fully backward compatible with existing source code.

The current semantic will change anyway (see also Matz
slides about Ruby 2.0), because it has some ambiguities.
for example:

arr = [1, 2, 3]
a, b, c = arr
#currently a == 1; b == 2; c == 3
#should be a == [1, 2, 3]; b == nil; c == nil

> 
> Would this solution solve the issue that your RCR solves ?
> 

I think a unify operator would be a different addition than
what I described in the RCR.

> Jean-Hugues
> 
> BTW: It is often said that operator = is unusal because it does
> not apply to an object. It could, we would need a BoundVariable
> class, close to a Binding, but referencing a specific variable.

Yes, I also think that would be nice.  Yet another RCR?

> 
> 
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