A quick scriptlet I whipped up for a friend tonight to allow them to retrieve an IP addess from a specific PPPoE interface.
Handy for updating DynDNS entries among other things.

:local wanip [/ip address get [/ip address find where interface=pppoe-wan] address];
:set wanip [:pick $wanip 0 ([:len $wanip]-3) ];
#whatever you want to do with said IP goes here
:put $wanip;

Obviously update pppoe-wan with your interface name 😉