I'm trying to add up digits one by one in two linked lists:
use List::*; enum List { Cons(i32, Box<List>), Nil, } impl List { fn stringify(&self) -> String { match *self { Cons(a, ref tail) => format!("{}, {}", a, tail.stringify()), Nil => format!("Nil") } } } fn dissolve_number(num: i32) -> List { match num { 0 => Nil, n => Cons(n % 10, Box::new(dissolve_number(n / 10))) } } fn add_with_inc(a: &List, b: &List, inc: bool) -> List { match (*a, *b, inc) { (Nil, Nil, false) => Nil, (Nil, Nil, true) => Cons(1, Box::new(Nil)), (Nil, Cons(j, ref btail), false) => Cons(j, Box::new(add_with_inc(&Nil, btail, false))), (Nil, Cons(j, ref btail), true) => Cons((j + 1) % 10, Box::new(add_with_inc(&Nil, btail, (j + 1) >= 10))), (Cons(i, ref atail), Nil, false) => Cons(i, Box::new(add_with_inc(atail, &Nil, false))), (Cons(i, ref atail), Nil, true) => Cons((i + 1) % 10, Box::new(add_with_inc(atail, &Nil, (i + 1) >= 10))), (Cons(i, ref atail), Cons(j, ref btail), false) => Cons((i + j) % 10, Box::new(add_with_inc(atail, btail, (i + j) >= 10))), (Cons(i, ref atail), Cons(j, ref btail), true) => Cons((i + j + 1) % 10, Box::new(add_with_inc(atail, btail, (i + j + 1) >= 10))) } } fn add(a: &List, b: &List) -> List { add_with_inc(a, b, false) } fn main() { let (la, lb) = (dissolve_number(123), dissolve_number(223)); let lsum = add(&la, &lb); println!("{}", lsum.stringify()); } The compiler complains:
error[E0507]: cannot move out of borrowed content --> add_two_numbers.rs:25:12 | 25 | match (*a, *b, inc) { | ^^ cannot move out of borrowed content error[E0507]: cannot move out of borrowed content --> add_two_numbers.rs:25:16 | 25 | match (*a, *b, inc) { | ^^ cannot move out of borrowed content After searching, I discovered that the error was due to dereferencing a borrowed variable. Is there any way to obtain a read-only copy of List from a reference?
I modified the code a bit, but I still can't get it working...
fn add_with_inc(a: &List, b: &List, inc: bool) -> List { match (a, b, inc) { (ref Nil, ref Nil, false) => Nil, (ref Nil, ref Nil, true) => Cons(1, Box::new(Nil)), (ref Nil, ref Cons(j, ref btail), false) => Cons(j, Box::new(add_with_inc(&Nil, btail, false))), (ref Nil, ref Cons(j, ref btail), true) => Cons((j + 1) % 10, Box::new(add_with_inc(&Nil, btail, (j + 1) >= 10))), (ref Cons(i, ref atail), ref Nil, false) => Cons(i, Box::new(add_with_inc(atail, &Nil, false))), (ref Cons(i, ref atail), ref Nil, true) => Cons((i + 1) % 10, Box::new(add_with_inc(atail, &Nil, (i + 1) >= 10))), (ref Cons(i, ref atail), ref Cons(j, ref btail), false) => Cons((i + j) % 10, Box::new(add_with_inc(atail, btail, (i + j) >= 10))), (ref Cons(i, ref atail), ref Cons(j, ref btail), true) => Cons((i + j + 1) % 10, Box::new(add_with_inc(atail, btail, (i + j + 1) >= 10))) } } 
