Super-dee-duper D features

[Andrei Alexandrescu (See Website For Email)]

Bill Baxter wrote:
> Andrei Alexandrescu (See Website For Email) wrote:
>> Frits van Bommel wrote:
>>> Andrei Alexandrescu (See Website For Email) wrote:
>>>> Sean Kelly wrote:
>>>>> What about:
>>>>>
>>>>>     foreach (i ; coll1) (j ; coll2)
>>>>>     {
>>>>>         if( true )
>>>>>             continue i;
>>>>>     }
>>>>>
>>>>> ie. allow 'continue' to accept labels to specify which collection 
>>>>> is iterated.  A 'continue' without labels would iterate both.
>>>>
>>>> I think that's a great idea, except that "continue to label" has the 
>>>> same syntax: http://digitalmars.com/d/statement.html#ContinueStatement
>>>
>>> Does that really matter? The compiler knows whether 'i' is a label or 
>>> a loop variable (presumably it can't be both at the same time?) so it 
>>> knows what to do. Note that the current "continue to label" wouldn't 
>>> help here since there's only one statement for a "double" iteration. 
>>> So the most natural way to specify which loop to continue would be to 
>>> specify the variable.
>>
>> That's a good point, but changing names could complicate maintenance.
>>
>>> By the way, would the new loop syntax allow more than two collections 
>>> to be simultaneously iterated?
>>> That would indicate the need for a "continue i, j" as well, to 
>>> specify multiple variables.
>>> On the other hand, with your proposed "continue foreach" clauses 
>>> after the main loop that would also require an exponential number of 
>>> those clauses for different sets of collections running out if you 
>>> want to handle all cases...
>>
>> That is correct. But it's not a real issue for a simple reason: the 
>> user would have to write the code. If they need to handle all cases, 
>> well, that's what they need to do one way or another. The foreach 
>> statement does not add anything to the equation.
> 
> Yes it does make a difference.  In the iterator case I can use all of D 
> to determine how to resolve the fact that there are different lengths of 
> data.  In the continue foreach scenario, I have only one recourse -- 
> write all the continute foreach's I need.
> 
> Consider I want to add N vectors of potentially different lengths 
> together.  With the right iterator solution I could get away with just:
> 
> foreach( iters,xs; multi_iter(a,b,c,d,e) )
>   foreach(it; iters)
>     if (!it.end) out[j] += xs[i];
> 
> With "continue foreach" I'm just going to give up and write a for loop.

That is correct. It's a limitation of "continue foreach" exactly because
it ties validity with syntactic scoping: the result gives a stronger
guarantee, but is inherently more restrictive.

I'll add that probably adding multiple vectors of different lengths 
would loop the other way around :o).


Andrei