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

Is this little feature worth the amount of language complexity increase it causes?

Bye,
bearophile