Convert a vector of strings to a deeply-nested JSON object

Question

I'm trying to get a deeply-nested JSON object from a vector of strings using the json crate:

fn main() {   
    let my_vec = ["foo", "bar", "baz", "foobar", "barfoo"];
    let mut curr_obj = object!();
    for i in 0..my_vec.len() {
        let name = my_vec[i];
        curr_obj = addObj(curr_obj, name);   
    }
}

fn addObj(mut obj: json::JsonValue, name: &str) -> json::JsonValue {
    obj[name] = json::JsonValue::new_object();
    let retob = obj[name];
    retob.to_owned() // is empty but should be obj["foo"] = object!();
}

The object is an empty one here. My desired output looks like this:

{
  "foo": {
    "bar": {
      "baz": {
        "foobar": {
          "barfoo": {}
        }
      }
    }
  }
}

I get the error

error[E0507]: cannot move out of indexed content
  --> src/main.rs:15:17
   |
15 |     let retob = obj[name];
   |                 ^^^^^^^^^
   |                 |
   |                 cannot move out of indexed content
   |                 help: consider using a reference instead: `&obj[name]`

addObj is not idiomatic Rust style, it should be snake_case: add_obj. — Shepmaster
– Shepmaster, Commented Oct 14, 2017 at 14:09

red75prime · Accepted Answer · 2017-10-14 08:17:03Z

3

It can be done with a little bit of magic.

fn main() {   
    let my_vec = ["foo","bar","baz","foobar","barfoo"];
    let mut curr_obj = object!();
    {
        let mut obj_ref = &mut curr_obj;
        for i in 0..my_vec.len() {
            let name = my_vec[i];
            obj_ref = &mut {obj_ref}[name]; // note the curly braces
        }
    }
    println!("{:?}", curr_obj);
}

Mutable reference is moved instead of being reborrowed.

answered Oct 14, 2017 at 8:17

red75prime

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4 Comments

red75prime Over a year ago

I remember there was similar question, but I can't find it right now.

user4815162342 Over a year ago

Can you please add more details (or a reference) about the magic of curly braces and the distinction between moving and reborrowing the mutable reference?

red75prime Over a year ago

I need to go. Search SO for reborrowing, there is good description.

abdoe Over a year ago

you sir - are a genius :)

Shepmaster · Accepted Answer · 2017-10-14 14:06:30Z

1

It's much simpler to use iterator methods:

#[macro_use]
extern crate json;

fn main() {
    let my_vec = ["foo", "bar", "baz", "foobar", "barfoo"];

    let result = my_vec.iter().rev().fold(object!(), |object, name| {
        object!(name => object)
    });

    println!("{}", json::stringify_pretty(result, 2));
}

Produces:

{
  "foo": {
    "bar": {
      "baz": {
        "foobar": {
          "barfoo": {}
        }
      }
    }
  }
}

answered Oct 14, 2017 at 14:06

Shepmaster

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