Using reduce() with tuples created by zip()

[bearophile]

> alias sum(T) = curry!(reduce!q{a + b}, cast(T)0);
>
> double euclidDistance(double[] xs, double[] ys)
> in {
>     assert(xs.length == ys.length);
> } body {
>     return zip(xs, ys)
>            .map!(xy => (xy[0] - xy[1]) ^^ 2)
>            .sum!double
>            .sqrt;
> }
>
> void main() {
>     auto xs = [0.0,  1.0, 0.1];
>     auto ys = [1.0, -1.0, 0.0];
>     euclidDistance(xs, ys).writeln;
> }

In D functions are written in camelCase like euclidDistance, 
instead of euclid_distance.

Function pre-conditions and post-conditions are generally better 
put in the pre and post part of functions.

I have defined a sum because it's a generally useful function, 
but it's a toy because it assumes cast(T)0 as the identity 
element of the monoid with the plus operator.

Bye,
bearophile