How can I convert toml-rs result to std::collections::HashMap

Question

I'm new to Rust and trying to build something simple to get going. I want to load data from a .toml file and use rustache to render out some text from it.

Rustache appears to take a HashMap as its data source, and I'm sure from looking at the toml-rs docs that I should be able to convert its Table and Array types to HashMaps and Vecs, and I suspect it's got something to do with Decoder, but I can't figure it out.

If somebody could provide a short example of how to do this I would be very grateful.

Answer 1

If your data structure has fixed known depth, then all you need is just to pass a correct type to toml::decode():

let value: toml::Value = toml::Value::Table(Parser::new(input).parse().unwrap());
let data: HashMap<String, Vec<u32>> = toml::decode(value).unwrap();

The code above would parse a document like

x = [1, 2, 3]
y = [4, 5, 6]

However, as far as I can see, rustache provides some kind of builder structure which supports arbitrary nesting. In that case you would need to "apply" toml::Value to rustache::HashBuilder. You don't need to use Decodable for this (though you probably can, with some newtypes) - you just need to write a couple of simple functions:

fn toml_into_hashbuilder<'a>(value: toml::Table, mut hb: rustache::HashBuilder<'a>) -> rustache::HashBuilder<'a> {
    for (k, v) in value {
        match v {
            toml::Value::String(s) => hb.insert_string(k, s),
            toml::Value::Integer(i) => hb.insert_int(k, i),
            toml::Value::Float(f) => hb.insert_float(k, f),
            toml::Value::Boolean(b) => hb.insert_bool(k, b),
            toml::Value::Datetime(s) => hb.insert_string(k, s),
            toml::Value::Array(arr) => hb.insert_vector(k, |vb| toml_into_vecbuilder(arr.clone(), vb)),
            toml::Value::Table(tbl) => hb.insert_hash(k, |hb| toml_into_hashbuilder(tbl.clone(), hb))
        }
    }
    hb
}

fn toml_into_vecbuilder<'a>(value: toml::Array, mut vb: rustache::VecBuilder<'a>) -> rustache::VecBuilder<'a> {
    for v in value {
        match v {
            toml::Value::String(s) => vb.push_string(s),
            toml::Value::Integer(i) => vb.push_int(i),
            toml::Value::Float(f) => vb.push_float(f),
            toml::Value::Boolean(b) => vb.push_bool(b),
            toml::Value::Datetime(s) => vb.push_string(s),
            toml::Value::Array(arr) => vb.push_vector(|vb| toml_into_vecbuilder(arr.clone(), vb)),
            toml::Value::Table(tbl) => vb.push_hash(|hb| toml_into_hashbuilder(tbl.clone(), hb))
        }
    }
    vb
}

let value: toml::Table = Parser::new(input).parse().unwrap();
let hb = toml_into_hashbuilder(value, rustache::HashBuilder::new());
let result = rustache::render_text(your_template, hb);

There are unfortunate clones when handling nested tables and arrays - this is a consequence of an issue in rustache. If it is fixed, clone() could be removed, and the closures should be made move then.

Answer 2

toml::decode is removed since toml 0.3. The new way is using toml::from_str

let data: HashMap<String, Vec<u32>> = toml::from_str(input).unwrap();