That is way over-engineered for something that could be done with a 10 lines of bash:

    diff2mail() {
        a=$($@)
        while true; do
            b=$($@)
            if [ "$a" != "$b" ]; then
                echo "$a -> $b" | mail -s "$@ changed" 'root@example.com'
                return
            fi
            sleep 1
        done
    }

    diff2mail ls /tmp

And for a lot of stuff it's even shorter than that:

    make && x="is done" || x="has failed"; echo "build $x" | mail root