學習Rust程式設計——自定義錯誤和Error特征

// todolist_parser/examples/basics.rs

extern crate todolist_parser;

use todolist_parser::TodoList;

fn main() {
    let todos = TodoList::get_todos("examples/todos");
    match todos {
        Ok(list) => println!("{:?}", list),
        Err(e) => {
            println!("{}", e.description());
            println!("{:?}", e)
        }
    }
}
           

// todolist_parser/src/error.rs

use std::error::Error;
use std::fmt;
use std::fmt::Display;

#[derive(Debug)]
pub enum ParseErr {
    Malformed,
    Empty
}

#[derive(Debug)]
pub struct ReadErr {
    pub child_err: Box<dyn Error>
}

// Required by error trait
impl Display for ReadErr {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Failed reading todo file")
    }
}

// Required by error trait
impl Display for ParseErr {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Todo list parsing failed")
    }
}

impl Error for ReadErr {
    fn description(&self) -> &str {
        "Todolist read failed: "
    }

    fn cause(&self) -> Option<&dyn Error> {
        Some(&*self.child_err)
    }
}

impl Error for ParseErr {
    fn description(&self) -> &str {
        "Todolist parse failed: "
    }

    fn cause(&self) -> Option<&Error> {
        None
    }
}           

// todolist_parser/src/lib.rs

//! This crate provides an API to parse list of todos

use std::fs::read_to_string;
use std::path::Path;

mod error;
use error::ParseErr;
use error::ReadErr;

use std::error::Error;

/// This struct contains a list of todos parsed as a Vec<String>
#[derive(Debug)]
pub struct TodoList {
    tasks: Vec<String>,
}

impl TodoList {
    pub fn get_todos<P>(path: P) -> Result<TodoList, Box<dyn Error>>
    where
    P: AsRef<Path>, {
        let read_todos: Result<String, Box<dyn Error>> = read_todos(path);
        let parsed_todos = parse_todos(&read_todos?)?;
        Ok(parsed_todos)
    }
}

pub fn read_todos<P>(path: P) -> Result<String, Box<dyn Error>>
where
    P: AsRef<Path>,
{
    let raw_todos = read_to_string(path)
        .map_err(|e| ReadErr {
            child_err: Box::new(e),
        })?;
    Ok(raw_todos)
}

pub fn parse_todos(todo_str: &str) -> Result<TodoList, Box<dyn Error>> {
    let mut tasks: Vec<String> = vec![];
    for i in todo_str.lines() {
        tasks.push(i.to_string());
    }
    if tasks.is_empty() {
        Err(ParseErr::Empty.into())
    } else {
        Ok(TodoList { tasks })
    }
}