You could define an `enum Default<T> = T`
On Friday, 25 January 2013, Michael Neumann wrote:
> Hi,
>
> I am getting the following error:
>
> msgpack.rs:545:0: 555:1 error: conflicting implementations for a trait
> msgpack.rs:545 pub impl<D: DecoderWithMap,
> msgpack.rs:546 K: serialize::Decodable<D>,
> msgpack.rs:547 V: serialize::Decodable<D>> ~[(K,V)]:
> serialize::Decodable<D> {
> msgpack.rs:548 static fn decode(&self, d: &D) -> ~[(K,V)] {
> msgpack.rs:549 do d.read_map |len| {
> msgpack.rs:550 do vec::from_fn(len) |i| {
> ...
> msgpack.rs:539:0: 543:1 note: note conflicting implementation here
> msgpack.rs:539 pub impl<D: DecoderWithMap, T> T: serialize::Decodable<D> {
> msgpack.rs:540 static fn decode(&self, d: &D) -> T {
> msgpack.rs:541 serialize::Decodable::decode(d as &serialize::Decoder)
> msgpack.rs:542 }
> msgpack.rs:543 }
>
>
> It's obvious that the two trait implementations conflict, as the one (for
> T) is
> more general as the other (for ~[(K,V)]). Is there anything I can do to
> fix it?
> I have found this discussion [1] but I see no solution to the problem.
>
> For msgpack, I want to support "maps". They are specially encoded, so I
> need a
> special Decoder (serialize::Decoder does not support maps in any way).
> Above I
> tried to extend the serialize::Decoder trait for read_map() and
> read_map_elt(),
> leading to DecoderWithMap:
>
> pub trait DecoderWithMap : serialize::Decoder {
> fn read_map<T>(&self, f: fn(uint) ⟶ T) ⟶ T;
> fn read_map_elt<T>(&self, _idx: uint, f: fn() ⟶ T) ⟶ T;
> }
>
> Then I tried to implement Decodable for ~[(K,V)] (which I want to use as a
> rust
> representation as a map; here I'd probably run into problems again as
> serializer defines a generic implementation for ~[] and for tuples...
> this at least I could solve by using a different type).
>
> Now I would need to reimplement Decodable for any type I use, so I tried
> to use the second generic trait implementation. But this is where it failed
> with a conflict.
>
> Would it be possible to "override" a standard (more generic) trait
> implementation,
> either implicitly (like C++ is doing) or explictly?
>
> Best,
>
> Michael
>
> [1]:
> https://github.com/mozilla/**rust/issues/3429<https://github.com/mozilla/rust/issues/3429>
>
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