I find myself in need of constructing an empty Result object. I tried takeNone!Result, but obviously the type Result doesn't appear to exist. Anyone any ideas?

-- 
Russel. ============================================================================= Dr Russel Winder      t: +44 20 7585 2200   voip: sip:russel.winder@ekiga.net 41 Buckmaster Road    m: +44 7770 465 077   xmpp: russel@winder.org.uk London SW11 1EN, UK   w: www.russel.org.uk  skype: russel_winder

On 04/17/2017 11:30 AM, Russel Winder via Digitalmars-d-learn wrote:
> I find myself in need of constructing an empty Result object. I tried
> takeNone!Result, but obviously the type Result doesn't appear to exist.
> Anyone any ideas?
>

(Not a complete solution; just sharing your pain.)

I had the same problem here:


http://ddili.org/ders/d.en/fibers.html#ix_fibers.Generator,%20std.concurrency

The solution I could use there is less than ideal and may not apply in all cases. I used typeof and was able to satisfy it with an empty delegate:

/* Returns an InputRange to the nodes of the tree. The
 * returned range is empty if the tree has no elements (i.e.
 * if 'root' is 'null'). */
auto byNode(const(Tree) tree) {
    alias RangeType = typeof(byNode(tree.root));

    return (tree.root
            ? byNode(tree.root)
            : new RangeType(() {}));    // ← Empty range
}

Ali

On Mon, Apr 17, 2017 at 12:05:19PM -0700, Ali Çehreli via Digitalmars-d-learn wrote: [...]
> auto byNode(const(Tree) tree) {
>     alias RangeType = typeof(byNode(tree.root));

Could this possibly be simplified to:

	alias RangeType = typeof(return);

?

Or does that cause a circular dependency that makes the compilation fail?


>     return (tree.root
>             ? byNode(tree.root)
>             : new RangeType(() {}));    // ← Empty range
> }
[...]


T

-- 
WINDOWS = Will Install Needless Data On Whole System -- CompuMan

On 04/17/2017 12:33 PM, H. S. Teoh via Digitalmars-d-learn wrote:
> On Mon, Apr 17, 2017 at 12:05:19PM -0700, Ali Çehreli via Digitalmars-d-learn wrote:
> [...]
>> auto byNode(const(Tree) tree) {
>>     alias RangeType = typeof(byNode(tree.root));
>
> Could this possibly be simplified to:
>
> 	alias RangeType = typeof(return);
>
> ?

Thank you. That's much better but only if I restructure the code to use an if statement:

auto byNode(const(Tree) tree) {
    if (tree.root) {
        return byNode(tree.root);
    }
    else {
        alias RangeType = typeof(return);
        return new RangeType(() {});
    }
}

That works because the return type of the function is the first return statement. (My original code had only one return and that was the problem.)

Ali

On Mon, 2017-04-17 at 15:10 -0700, Ali Çehreli via Digitalmars-d-learn wrote:
> alias RangeType = typeof(return);
>          return new RangeType(() {});


It appears the answer to my question is:

	return typeof(return)();

Thanks to Ali and T for setting on the right path.

-- 
Russel. ============================================================================= Dr Russel Winder      t: +44 20 7585 2200   voip: sip:russel.winder@ekiga.net 41 Buckmaster Road    m: +44 7770 465 077   xmpp: russel@winder.org.uk London SW11 1EN, UK   w: www.russel.org.uk  skype: russel_winder