Re: Memoization in compile-time

[Meta via Digitalmars-d-learn]

On Friday, 13 March 2015 at 12:49:48 UTC, Dennis Ritchie wrote:
On Friday, 13 March 2015 at 02:38:18 UTC, Rikki Cattermole 
wrote:
You could assign it to e.g. an enum. Or force it over using 
meta-programming.

And this code can be rewritten to D?

template <int n>
struct Factorial
{
    enum { value = n * Factorial<n - 1>::value };
};

template <>
struct Factorial<0>
{
    enum { value = 1 };
};

int main()
{
    constexpr auto x = Factorial<5>::value;
    constexpr auto y = Factorial<7>::value;
}

You can translate it directly:

template Factorial(int n)
{
    static if (n == 0)
    {
        enum Factorial = 1;
    }
    else static if (n > 0)
    {
        enum Factorial = n * Factorial!(n - 1);
    }
    else
    {
        static assert(false, "n cannot be negative");
    }
}

int main()
{
    //Calculated at compile time
    auto x = Factorial!5;
    auto y = Factorial!7;
}




However, it's much easier to just write a function and decide 
whether you want to calculate its value at runtime or compile 
time.

int factorial(int n)
in
{
    assert(n >= 0, "n cannot be 0");
}
body
{
    return n == 0 ? 1 : n * factorial(n - 1);
}

void main()
{
    //Evaluated at compile time
    enum x = factorial(5);

    //Evaluated at runtime
    auto y = factorial(7);
}