The new ?? and ??? operators

[Stewart Gordon]

"Arlen Albert Keshabyan" <arlen.albert at gmail.com> wrote in message
news:fd611h$4co$1 at digitalmars.com...
> It would be the sugar syntactic to add '??' operator to D.
> Consider the example code:
>
> string error_message = getErrorMessage() ??  "no errors";
> A a = x.getPreparedAObject() ??  y.getPreparedAObject() ??  new A();
>
> This operator is supposed to do the same thing as C# does: lvalue
> evaluates to the first non-null value or to null if all rvalues
> contains null.

So effectively, it works like || in JavaScript and the like.  I guess the 
return type of a ??  expression would be determined by the same rules that 
govern that of a ConditionalExpression.  The expression would evaluate to 
the first subexpression whose value when implicitly converted to a boolean 
is true.  Otherwise ...  to the .init of the return type?

> This operator might be extended to '???' to evaluate to a value
> that conforms to some conditions.  The lvalue gets the first rvalue
> that evaluates to true (evaluation goes from left to right).  If no
> conditions evaluates to true then the lvalue stays unchanged.  If
> no conditions are given explicitly then those conditions evaluates
> to true (so, the best place for them is at the end of a sequence).
<snip>

I don't really like this:
- It would cause the semantics of the = operator to depend on the form of 
the RHS.
- What if the ??? expression isn't the RHS of an = operator?
- The null case of this operator doesn't match semantically as they're 
normally expected to.  (I'm not sure if there's any better way to word 
this.)  To see what I mean, compare the meanings of

a = b ??? c ??? d;
a = b ??? c;
a = b;

Stewart.

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