Add pop method to json Decoder

[steveklabnik]

Issue by yaitskov
Thursday Sep 18, 2014 at 20:14 GMT

For earlier discussion, see rust-lang/rust#17377

This issue was labelled with: in the Rust repository

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Now it's impossible to implement Decodable for a map with different value types
because read_* methods lose the stack head anyway.
Decodable doesn't have a method returning the stack head as-is without any type casting.

Let's look a case where Json dictionary has values with int and String types.

extern crate serialize;
use serialize::{Decodable, Decoder};
use serialize::json::{Json, Boolean, String, Null, I64, U64, F64};
use serialize::json;

#[deriving(Show)]
struct Primitive(Json);

impl<S: Decoder<E>, E> Decodable<S, E> for Primitive {
    fn decode(decoder: &mut S) -> Result<Primitive, E> {
        match decoder.pop() {      
            n @ I64(_)    => Primitive(n),
            n @ U64(_)    => Primitive(n),
            n @ F64(_)    => Primitive(n),
            s @ String(_) => Primitive(s),
            bad           => fail!("bad {}", bad)
       }
    }
}

[oli-obk]

Note: Decodable is not a generic trait. The decode function is generic instead.
Note2: You can derive the Decodable implementation for BTreeMap<String, Option<String>> or any other enum type that derives Decodable as Value.

Obviously Note2 does not work to decode arbitrary json formats, only those that also were encoded by a derived Encodable trait.

As the decoder could also be any other decoder (like a binary decoder), the type of the field to read might not be available. Implementing a generic pop is therefor not possible. Making the pop method from json::Decoder public and waiting for negative trait bounds might work (rust-lang/rfcs#586).

[alexcrichton]

I'm going to close this now that this crate is deprecated in favor of serde. We're discontinuing feature development in rustc-serialize but will still continue to merge bug fixes if they arise.