Read JSON file using from_reader manually

---

cargo run ../person.json

examples/json/person.json

{
    "fname": "Foo",
    "lname": "Bar",
    "year": 1992,
    "height": 178.2,
    "married": true,
    "numbers": [23, 19, 42],
    "children": [
        {
            "name": "Alpha",
            "birthdate": 2020
        },
        {
            "name": "Beta",
            "birthdate": 2022
        }
    ]
}

examples/json/read-json-from-reader-manually/Cargo.toml

[package]
name = "json-read-from-reader"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
serde = { version = "1.0", features = ["derive"] }
serde_json = "1.0.97"

examples/json/read-json-from-reader-manually/src/main.rs

use std::fs::File;

fn main() {
    let filename = get_filename();

    let data = match File::open(&filename) {
        Ok(file) => {
            let data: serde_json::Value =
                serde_json::from_reader(&file).expect("JSON parsing error");
            data
        }
        Err(error) => {
            eprintln!("Error opening file {}: {}", filename, error);
            std::process::exit(1);
        }
    };
    println!("{:#?}", &data);
    assert_eq!(data.get("fname").unwrap().as_str().unwrap(), "Foo");

    assert_eq!(data["lname"].as_str().unwrap(), "Bar");
    assert_eq!(data["height"].as_f64().unwrap(), 178.2);
    assert_eq!(data["year"].as_u64().unwrap(), 1992);
    assert_eq!(data["numbers"].as_array().unwrap().len(), 3);
    assert_eq!(data["numbers"][0].as_u64().unwrap(), 23);
    assert_eq!(data["married"].as_bool().unwrap(), true); // maybe better to use assert!
    assert_eq!(data["children"].as_array().unwrap().len(), 2);
    assert_eq!(data["children"][0]["name"].as_str().unwrap(), "Alpha");
}

fn get_filename() -> String {
    let args: Vec<String> = std::env::args().collect();
    if args.len() != 2 {
        eprintln!("Usage: {} FILENAME", args[0]);
        std::process::exit(1);
    }
    args[1].to_owned()
}

Object {
    "children": Array [
        Object {
            "birthdate": Number(2020),
            "name": String("Alpha"),
        },
        Object {
            "birthdate": Number(2022),
            "name": String("Beta"),
        },
    ],
    "fname": String("Foo"),
    "height": Number(178.2),
    "lname": String("Bar"),
    "married": Bool(true),
    "numbers": Array [
        Number(23),
        Number(19),
        Number(42),
    ],
    "year": Number(1992),
}