rust-roguelike-tutorial/chapter-06-monsters/src/monster_ai_system.rs

extern crate specs;
use specs::prelude::*;
use super::{Viewshed, Monster, Name};
extern crate rltk;
use rltk::{Point};

pub struct MonsterAI {}

impl<'a> System<'a> for MonsterAI {
    type SystemData = ( ReadExpect<'a, Point>,
                        ReadStorage<'a, Viewshed>, 
                        ReadStorage<'a, Monster>,
                        ReadStorage<'a, Name>);

    fn run(&mut self, data : Self::SystemData) {
        let (player_pos, viewshed, monster, name) = data;

        for (viewshed,_monster,name) in (&viewshed, &monster, &name).join() {
            if viewshed.visible_tiles.contains(&*player_pos) {
                println!("{} shouts insults", name.name);
            }
        }
    }
}