add pointer examples.

rust/pointers.rs

@@ -0,0 +1,86 @@
extern crate debug;
mod wrapped {
    fn get_wrapped(v: &int) -> int {
        *v
    }
    #[deriving(Show)]
    struct Point {x: f64, y: f64}
    fn get_wrapped_struct(p: &Point) -> Point {
        *p
    }
    pub fn run() {
        println!("\nwrapped:");
        // mhh... i thought this would "move" the ownership, but somehow it doesn't. because it's immutable?
        // by the way, how is immutability defined? we can have mutable variables, but also mutable pointers,
        // what does that mean?
        println!("get_wrapped(&1) = {}", get_wrapped(&1));
        // even this does work, so what doesn't?
        let point = Point{x: 3.1, y: 4.7};
        println!("get_wrapped_struct(&Point{{x = 3.1, y = 4.7}}) = {}", get_wrapped_struct(&point));
    }
}
mod list_with_box {
    #[deriving(Show)]
    pub enum List<A> {
        Nil,
        Cons(A, Box<List<A>>)
    }
    // this could already be "ok", if we accept that the argument will be copied just to match on it. i think rust uses `memcpy` for that, so it should be quite fast, even if generally "bad".
    pub fn cons<A>(x: A, xs: List<A>) -> List<A> {
        match xs {
            Nil => Cons(x, box Nil),
            _   => Cons(x, box xs)
        }
    }
    pub fn run() {
        println!("\nlist_with_box:");
        let l: List<int> = cons(1, cons(2, Nil));
        println!("l = {}", l);
        println!("cons(3, box l) = {:?}", cons(3, l));
    }
}
mod list_with_ref {
    #[deriving(Show)]
    pub enum List<'a, A: 'a> {
        Nil,
        Cons(A, &'a List<'a A>) // are there alternatives to boxing? (i think not -- Rc, Arc, etc. *are* boxes. right?)
    }
    // we want to pass a reference to the list, pattern match on it and then return a new list that references the old one.
    pub fn cons<'a, A>(x: A, xs: &'a List<A>) -> List<'a, A> {
        match *xs {
            Nil => Cons(x, xs),
            ref xs  => Cons(x, xs)
        }
    }
    pub fn run() {
        println!("\nlist_with_ref:");
        //let l: &List<int> = &Cons(1, &Cons(2, &Cons(3, &Nil)));
        let nil = &Nil;
        let l: &List<int> = &cons(1, nil);
        println!("l = {:?}", l);
        println!("lwr_cons_with_ref(1, l) = {:?}", cons(1, l));
    }
}
pub fn main() {
    wrapped::run();
    list_with_box::run();
    list_with_ref::run();
}