static foreach

[bearophile]

Navin Kumar:

> I was going over the D tutorials on tuples and noticed that it's possible to do a "foreach" across the tuple items.  I'm assuming this is a compile-time foreach, is that correct?

Correct.


> I would like to solve this problem with a compile-time foreach; how do I do so?  "static foreach" is not valid syntax in D.

In my dlibs1 I have:

template Range(int stop) {
    static if (stop <= 0)
        alias Tuple!() Range;
    else
        alias Tuple!(Range!(stop-1), stop-1) Range;
}

/// ditto
template Range(int start, int stop) {
    static if (stop <= start)
        alias Tuple!() Range;
    else
        alias Tuple!(Range!(start, stop-1), stop-1) Range;
}

/// ditto
template Range(int start, int stop, int step) {
    static assert(step != 0, "Range: step must be != 0");

    static if (step > 0) {
        static if (stop <= start)
            alias Tuple!() Range;
        else
            alias Tuple!(Range!(start, stop-step, step), stop-step) Range;
    } else {
        static if (stop >= start)
            alias Tuple!() Range;
        else
            alias Tuple!(Range!(start, stop-step, step), stop-step) Range;
    }
} // End Range!(a,b,c)


With that you can unroll loops and push multiple cases inside a switch statement.
With a little more work you can unroll in cases where you don't know the number of loops, using a modulus and vision approach.

The main problem, baside a bit of bloat at compile time, is that such forach(i; Range!(...)) doesn't work outside functions, as a good static foreach has to.

Bye,
bearophile