Hamming distance

import std.stdio;
import std.algorithm;  // count()
import std.range;  // zip()

int hamming_dist(const string s1, const string s2) pure
{
    if (s1.length != s2.length) {
        return -1;
    }
    // else
    // return cast(int) zip(s1, s2).count!((a, b) => a != b);  // error, unpacking doesn't work here
    return cast(int) zip(s1, s2).count!(t => t[0] != t[1]);
}

void main()
{
    const s1 = "toned";
    const s2 = "roses";

    const dist = hamming_dist(s1, s2);
    writeln(dist); // 3
}