Maps and round trips in Rust

Serializing a map, then reading it back into the same type.

use serde::{Deserialize, Serialize};
use std::collections::BTreeMap;

#[derive(Debug, Serialize, Deserialize, PartialEq)]
struct Progress {
    tours: BTreeMap<String, u32>,
}

fn main() -> Result<(), serde_json::Error> {
    let mut tours = BTreeMap::new();
    tours.insert("traits".to_string(), 15);
    tours.insert("basics".to_string(), 39);

    let before = Progress { tours };
    let json = serde_json::to_string(&before)?;
    println!("{json}");

    let after: Progress = serde_json::from_str(&json)?;
    println!("equal: {}", before == after);
    Ok(())
}

How it works

1. A HashMap becomes a JSON object; keys must be strings.
2. BTreeMap keeps the keys sorted in the output.
3. A round trip is the cheapest test of a model.

Keywords and builtins used here

• BTreeMap
• Ok
• Progress
• Result
• String
• fn
• let
• main
• mut
• struct
• u32
• use

The run, in numbers

Lines

21

Characters to type

530

Tokens

147

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 88 seconds.

Type this snippet

Step 3 of 3 in Deriving, step 3 of 11 in Serde & JSON.

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