Using List<T> in Rust

Question

I'm just starting in Rust and trying to write some basic algorithms to get a feel for the language. My attempt here is to write a function that given an int will convert it into binary form represented by a linked list of bool values with true for 1 and false for 0 (if there's a better "bit" type I'm open to that suggestion as well).

My current implementation is as follows:

fn to_base_2(num: int) -> List<bool> {
    fn to_base_2_acc(num: int, acc: List<bool>) -> List<bool> {
        if num == 0 {
            return acc;
        } else {
            let bit = (num % 2) == 0;
            let new_acc = Cons(bit, acc);
            return to_base_2_acc(num / 2, new_acc);
        }
    }

    to_base_2_acc(num, Nil);
}

This code fails to compile with:

main.rs:20:28: 20:31 error: mismatched types: expected `@extra::list::List<bool>` but found `extra::list::List<bool>` (expected @-ptr but found enum extra::list::List)
main.rs:20              let new_acc = Cons(bit, acc);
                                                ^~~

However updating the code to be managed by dropping @ in front of List results in the following:

main.rs:15:34: 15:45 error: The managed box syntax is being replaced by the `std::gc::Gc` and `std::rc::Rc` types. Equivalent functionality to managed trait objects will be implemented but is currently missing.
main.rs:15      fn to_base_2_acc(num: int, acc: @List<bool>) -> List<bool> {
                                                ^~~~~~~~~~~
main.rs:15:34: 15:45 note: add #[feature(managed_boxes)] to the crate attributes to enable
main.rs:15      fn to_base_2_acc(num: int, acc: @List<bool>) -> List<bool> {
                                                ^~~~~~~~~~~
main.rs:25:21: 25:25 error: The managed box syntax is being replaced by the `std::gc::Gc` and `std::rc::Rc` types. Equivalent functionality to managed trait objects will be implemented but is currently missing.
main.rs:25      to_base_2_acc(num, @Nil);
                                   ^~~~
main.rs:25:21: 25:25 note: add #[feature(managed_boxes)] to the crate attributes to enable
main.rs:25      to_base_2_acc(num, @Nil);
                                   ^~~~

This is using rustc 0.9-pre. Do linked lists not work in this version of the compiler?

With the landing of github.com/mozilla/rust/pull/10576 , Gc<T> is replacing the @ syntax, but adding #[feature(managed_boxes)] in top of your code should make that error go. — snf
– snf, Commented Dec 26, 2013 at 22:03

Lily Ballard · Accepted Answer · 2013-12-27 18:12:51Z

3

You need the @'s because extra::list::List is defined such that Cons requires an @List<T> as the second value.

However, as you found out, trying to add @'s throws a feature gate error. As snf commented, you can add

#[feature(managed_boxes)];

to the top of the root crate file to enable the usage of @.

answered Dec 27, 2013 at 18:12

Lily Ballard

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2 Comments

T. Stone Over a year ago

Are "managed_boxes" an old feature going away or a new feature coming in?

Lily Ballard Over a year ago

@T.Stone: They're an old feature going away. They're being replaced with library types std::gc::Gc<T> and std::rc::Rc<T>.

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