On 17 Sep 2014, at 23:33, Sean McArthur <smcart...@mozilla.com> wrote:
> On Wed, Sep 17, 2014 at 2:26 PM, Evan Davis <cptr...@gmail.com> wrote:
>
> The problem is that you're trying to use a trait as a type.
>
> That shouldn't be a problem. You can use a `&mut Trait`, and you'll get
> dynamic dispatch.
You cannot mix dynamic dispatch with generic methods. (This is because we
implement generic methods by monomorphization, i.e. by creating separate copies
of the implementation for each instantiation -- but with dynamic dispatch, we
do not know a priori what all of the calls to the method will be.)
I would link a relevant portion of the Rust manual here, but I cannot actually
find documentation of this restriction.
----
Pete’s original question was actually answered in his followup email. It is
true that you cannot do:
> fn print_numbers(r: &mut Rng) {
> for _ in range(0u, 10) {
> println!("{}", r.gen::<uint>());
> }
> }
due to the restriction I described above, but you can do:
> fn print_numbers<R: Rng> (r: &mut R) {
> for _ in range(0u, 10) {
> println!("{}", r.gen::<uint>());
> }
> }
where the trait is now being used as a bound on a generic type, rather than as
the basis of an object type.
Cheers,
-Felix
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