Template question

[Daniel Keep]

Michiel wrote:
> I'm not yet familiar enough with the template system to do this.
> 
> I wrote the following function:
> 
> ----------------------------------------
> Set!(T[0]) set(T ...)(T elements) {
>     auto result = new Set!(T[0]);
> 
>     foreach (element; elements) {
>         result.add(element);
>     }
> 
>     return result;
> }
> ----------------------------------------
> 
> This gives a nice set literal syntax with implicit type deduction.
> 
> auto x = set(4, 5, 6, 99);
> 
> But the user of the function should also have the option to use it like
> this:
> 
> auto x = set!(int);
> 
> To create the empty set (that can hold int). How can I implement this?
> 
> Thanks!

SetOf!(T) set(T...)(T elements)
{
    auto result = new SetOf!(T);

    foreach( element ; elements )
        result.add(element);

    return result;
}

template SetOf(T...)
{
    static if( is( typeof(T[0]) ) )
        alias Set!(typeof(T[0])) SetOf;
    else
        alias Set!(T[0]) SetOf;
}

Obviously, this only works if Set!(T) expects "T" to be an actual type.

Note that I haven't tried the above.  This should *theoretically* work
since taking the type of a type makes no sense (hence, the is expression
should fail), whilst taking the type of a value does.

If it doesn't work, blame it on just having woken up :P

> PS: I would have liked to skip the type altogether for the empty set,
> but the problem appears when you want to put something inside that set.
> I've considered using a dummy type and to somehow construct the correct
> set-type after the first insert, but it wouldn't be consistent with
> normal sets and altogether too much trouble.

Bad idea; the more you can get fixed at compile-time, the better.  For
instance, std.boxer.Box can store any type you want.  Putting things
into a box is easy enough,

  auto x = box(42);

But taking them back out is a bit ugly.

  auto y = unbox!(int)(x);

The reason for this is that the compiler needs to know what the result
type of "unbox" is at compile-time, so you need to tell it.

	-- Daniel

-- 
Unlike Knuth, I have neither proven or tried the above; it may not even
make sense.

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