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## boxing_todo
### Instructions
The objective is to create an API to parse a list of _todos_ that are organized in a JSON file. You must handle all possible errors in a multiple error system.
Organization of the JSON file:
```json
{
"title": "TODO LIST FOR PISCINE RUST",
"tasks": [
{ "id": 0, "description": "do this", "level": 0 },
{ "id": 1, "description": "do that", "level": 5 }
]
}
```
#### err.rs
Create a module in a file named **err.rs** which handles the boxing of errors.
This module must implement an `enum` called `ParseErr` which will take care of the parsing errors. It must have the following elements:
- `Empty`
- `Malformed`: which has a dynamic boxed error as element
A structure called `ReadErr` which will take care of the reading errors, with an element called `child_err` of type `Box<dyn Error>`.
For each data structure, you will have to implement a function called `fmt` for the `Display` trait. It should write the message **"Fail to parse todo"** in the case of any parsing error. Otherwise, it should write the message **"Fail to read todo file"**.
For the `Error` trait, the following functions (methods) have to be implemented:
- `source` which returns an `Option` with the error:
- For the `ReadErr`, it must return the option with the error.
- For the `ParseErr`, it will return an option which is `None` if the tasks are **empty**, and the error if the parsing is **malformed**.
#### lib.rs
In the **lib** file you will have to implement a **function** called `get_todo` which receives a string and returns a `Result` which can be the structure `TodoList` or a boxing error. This **function** must be able to deserialize the json file.
Basically it must parse and read the JSON file and return the `TodoList` if everything is fine, otherwise it returns the error.
### Expected Functions
For **err.rs**
```rust
use std::fmt;
use std::fmt::Display;
use std::error::Error;
pub enum ParseErr {
// expected public fields
}
// required by error trait
impl Display for ParseErr {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
}
}
pub struct ReadErr {
// expected public fields
}
// required by error trait
impl Display for ReadErr {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
}
}
impl Error for ParseErr {
fn source(&self) -> Option<&(dyn Error + 'static)> {
}
}
impl Error for ReadErr {
fn source(&self) -> Option<&(dyn Error + 'static)> {
}
}
```
for **lib.rs**
```rust
mod err;
use err::{ ParseErr, ReadErr };
pub use json::{parse, stringify};
pub use std::error::Error;
#[derive(Debug, Eq, PartialEq)]
pub struct Task {
pub id: u32,
pub description: String,
pub level: u32,
}
#[derive(Debug, Eq, PartialEq)]
pub struct TodoList {
pub title: String,
pub tasks: Vec<Task>,
}
impl TodoList {
pub fn get_todo(path: &str) -> Result<TodoList, Box<dyn Error>> {
}
}
```
### Usage
Here is a program to test your function.
You can create some _todos_ yourself, inside the boxing_todo file in order to test it. The JSON structure can be found above.
```rust
use boxing_todo::TodoList;
fn main() {
let todos = TodoList::get_todo("todo.json");
match todos {
Ok(list) => println!("{:?}", list),
Err(e) => {
println!("{}{:?}", e.to_string(), e.source());
}
}
let todos = TodoList::get_todo("todo_empty.json");
match todos {
Ok(list) => println!("{:?}", list),
Err(e) => {
println!("{}{:?}", e.to_string(), e.source());
}
}
let todos = TodoList::get_todo("malformed_object.json");
match todos {
Ok(list) => println!("{:?}", list),
Err(e) => {
println!("{}{:?}", e.to_string(), e.source());
}
}
}
```
And its output:
```console
$ cargo run
TodoList { title: "TODO LIST FOR PISCINE RUST", tasks: [Task { id: 0, description: "do this", level: 0 }, Task { id: 1, description: "do that", level: 5 }] }
Fail to parses todoNone
Fail to parse todo Some(Malformed(UnexpectedCharacter { ch: ',', line: 2, column: 18 }))
$
```
### Notions
- [Module std::fmt](https://doc.rust-lang.org/std/fmt/)
- [JSON](https://docs.rs/json/0.12.4/json/)
- [Boxing errors](https://doc.rust-lang.org/stable/rust-by-example/error/multiple_error_types/boxing_errors.html)
- [Returning Traits with dyn](https://doc.rust-lang.org/stable/rust-by-example/trait/dyn.html)