Template limits

[bearophile]

Walter Bright:
> The problem is that enough recursion will blow up the stack in the 
> compiler. I set a limit below that. But, naturally, for any limit I set 
> someone will try to exceed it.

I accept that some limits exist. For many situations a template nesting of 1000 is plenty. My problems were:
1) The limit of D1 seems different of the limit of D2, so a program I have written for D1 doesn't work with D2.
2) Of course I have first tried to write it with CTFE, but I have failed. First I have tried the simple solution:

int[] genSquares(int n) {
    int[] result;
    for (int i = 0; i < n; i++) {
        int k = i;
        int m;
        while (k) {
            int digit = k % 10;
            k /= 10;
            m += digit * digit;
        }
        result ~= m;
    }
    return result;
}

Then seeing it fail (I cast it into a fixed sized array at compile time), I have tried creating a fixed sized array, but functions can't return them yet. So I have tried to wrap the static array with a struct, but Don has shown this doesn't currently work yet:

struct S(int N) { int[N] a; }

S!(N) genSquares(int N)() {
    S!(N) s;
    for (int i = 0; i < N; i++) {
        int n = i;
        int m = 0;
        while (n) {
            int digit = n % 10;
            n /= 10;
            m += digit * digit;
        }
        s.a[i] = m;
    }

    return s;
}
void main() {
    const int CHUNK = 1000;
    static const auto squares = genSquares!(CHUNK)().a;
}

So I have created the mixed template/ctfe version I have shown in my original post.
So far I have written tons of (sometimes quite complex) templates in D :-)

Max Samukha then has gently offered me a working solution, but it's not obvious, it uses a dynamic array, but builds it in a more functional way.

Bye,
bearophile