auto rk4(alias f, T)(T x, int N, T dt) {

    import std.range : iota;

    auto dt2 = dt/2;

    auto dt6 = dt/6;

    auto t = 0.0;

    foreach (n; iota(0, N)) {

        auto s1 = f(t, x);

        auto s2 = f(t + dt2, x + dt2*s1);

        auto s3 = f(t + dt2, x + dt2*s2);

        auto s4 = f(t + dt,  x + dt *s3);

        x = x + dt6*(s1 + 2.0*s2 + 2.0*s3 + s4);

    }

    return x;

}

​

int main(string[] args) {

    import std.stdio : writeln;

    writeln(rk4!((t, x) => x*(1.0-x))(0.5, 100_000_000, 0.01));  // 1d logistic ODE

    return 0;

}