Converting json to TreeMap<String, String> - json

Question

I want to convert json to TreeMap in a simple way, here's my attempt:
extern crate serialize;
use serialize::json;
use serialize::json::ToJson;
use serialize::json::Json;
use std::collections::TreeMap;
fn main() {
let mut tree_map1 = TreeMap::new();
tree_map1.insert("key1".to_string(), "val1".to_json());
tree_map1.insert("key2".to_string(), "val2".to_json());
//... and so on, the number of keys aren't known
let json1 = json::Object(tree_map1);
let mut tree_map2 = TreeMap::new();
for (k, v) in json1.iter() { //impossible to iterate
tree_map2.insert(k.to_string(), v.to_string());
}
}
UPDATE:
How do I convert TreeMap<String, json::Json> to TreeMap<String, String> ?
let json1 = get_json1(); // json made of TreeMap<String, json::Json>
let res = match json1 {
json::Object(json2) => json2.map(|k, v| ??? ),
_ => panic!("Error")
}

Answer 1

Here is a demonstration of safely converting Json into TreeMap<String, String>:
use serialize::json::Json;
use std::collections::TreeMap;
fn extract_string_map(json: Json) -> Result<TreeMap<String, String>, Json> {
let json = match json {
Json::Object(json) => json,
_ => return Err(json),
};
if !json.iter().all(|(_k, v)| v.is_string()) {
return Err(Json::Object(json));
}
Ok(json.into_iter().map(|(k, v)| (k, match v {
Json::String(s) => s,
_ => unreachable!(),
}))
.collect())
}
This demonstrates a principle of avoiding panic-prone behaviour, for this cannot fail. Also it doesn’t lose any data—should the data not fit the format, the original data is returned intact for the caller to decide what to do with it, without having needed to clone data at any point.
(As a point of curiosity, I think that this restructuring of a TreeMap is going to be fairly inefficient, requiring more rebalancing of the tree than it should, because of the keys being given in order. For performance it’d be great to have a value-changing method for a TreeMap, consuming self and producing a new type more efficiently.)

Answer 2

json::Object is an enum variant which contains TreeMap inside it. So in order to get a TreeMap from it, you just need to unwrap it:
let json1 = json::Object(tree_map1);
let tree_map2 = match json1 {
json::Object(tm) => tm,
_ => unreachable!()
};
This will consume json1. If you don't want it, you need to clone the map:
let tree_map2 = match json1 {
json::Object(ref tm) => tm.clone(),
_ => unreachable!()
};
The latter can be rewritten less noisily with as_object() method:
let tree_map2 = json1.as_object().unwrap().clone();
If you need to obtain TreeMap<String, String> from TreeMap<String, Json> which is contained inside Object variant, you need to convert Json to String somehow. If you know in advance that all values are JSON strings, you can use pattern matching again:
let tree_map2 = match json1 {
json::Object(tm) => tm.into_iter().map(|(k, v)| (k, match v {
json::String(s) => s,
_ => unreachable!()
})).collect(),
_ => unreachable!()
};