Private default arguments?

[Gor Gyolchanyan]

I like the idea. I really don't like having the overhead of inner functions
just for compile-time enforcement.


On Fri, Dec 28, 2012 at 2:58 PM, bearophile <bearophileHUGS at lycos.com>wrote:

> Often recursive functions need some "bookkeeping" default arguments that
> aren't meant to be used by the user, they are meant to be used only by
> recursive calls:
>
>
> void radixSort(uint[] items, in uint shiftBits=24) {
>     ...
>     if (shiftBits > 0) {
>         ...
>         radixSort(array[...], shiftBits - 8);
>     }
> }
> void main() {
>     auto array = new uint[n];
>     ...
>     array.radixSort();
> }
>
>
>
> So this call is a bug:
>
> void main() {
>     ...
>     array.radixSort(26);
> }
>
>
> To avoid bugs and to not expose such private arguments I sometimes define
> an inner function (or a private function in a struct/class). Now the only
> argument of the outer function is 'items', and no mistakes can happen using
> radixSort2():
>
>
> void radixSort2(uint[] items) {
>     void radix(in uint shiftBits=24) {
>         ...
>         if (shiftBits > 0) {
>             ...
>             radixSort(array[...], shiftBits - 8);
>         }
>     }
>     radix();
> }
>
>
> This has some disadvantages.
>
> An alternative idea (that I maybe I proposed years ago in a weaker form)
> is to introduce 'private' default arguments (they must have a default
> value):
>
>
> void radixSort3(uint[] items, private in uint shiftBits=24) {
>     ...
>     if (shiftBits > 0) {
>         ...
>         radixSort(array[...], shiftBits - 8);
>     }
> }
>
>
> The 'private' means that only radixSort3 is allowed to set a shiftBits
> argument value. So this is reported as compilation error:
>
> void main() {
>     ...
>     array.radixSort(26);
> }
>
>
> A more detailed proposal, almost a DEP:
> http://d.puremagic.com/issues/**show_bug.cgi?id=9229<http://d.puremagic.com/issues/show_bug.cgi?id=9229>
>
> Is this little feature worth the amount of language complexity increase it
> causes?
>
> Bye,
> bearophile
>



-- 
Bye,
Gor Gyolchanyan.
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