我正在使用提供基于树的数据结构的第三方库(这意味着我必须使用它as is
),API函数返回Result<Data, Error>
。就我而言,我必须进行一些顺序调用并将错误转换为应用程序的内部。
例如:
use std::error::Error;
use std::fmt;
pub struct Tree {
branches: Vec<Tree>
}
impl Tree {
pub fn new(branches: Vec<Tree>) -> Self {
Tree { branches }
}
pub fn get_branch(&self, id: usize) -> Result<&Tree, TreeError> {
self.branches.get(id).ok_or(TreeError { description: "not found".to_string() })
}
}
#[derive(Debug)]
pub struct TreeError {
description: String,
}
impl Error for TreeError {
fn description(&self) -> &str {
self.description.as_str()
}
}
impl fmt::Display for TreeError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
self.description.fmt(f)
}
}
#[derive(Debug)]
pub struct MyAwesomeError {
description: String,
}
impl MyAwesomeError {
pub fn from<T: fmt::Debug>(t: T) -> Self {
MyAwesomeError {
description: format!("{:?}", t),
}
}
}
impl Error for MyAwesomeError {
fn description(&self) -> &str {
&self.description
}
}
impl fmt::Display for MyAwesomeError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
self.description.fmt(f)
}
}
如果我编写类似的代码:
pub fn take_first_three_times(tree: &Tree) -> Result<&Tree, MyAwesomeError> {
let result = tree.get_branch(0)
.map(|r| r.get_branch(0))
.map(|r| r.map(|r| r.get_branch(0)));
...
}
result
的类型将是Result<Result<Result<Tree, TreeError>, TreeError>, TreeError>
。当然,我不想通过match
的级联来处理错误。
我可以编写一个适应API接口的“内部”函数,并处理基本函数级别的错误:
fn take_first_three_times_internal(tree: &Tree) -> Result<&Tree, TreeError> {
tree.get_branch(0)?.get_branch(0)?.get_branch(0)
}
pub fn take_first_three_times(tree: &Tree) -> Result<&Tree, MyAwesomeError> {
take_first_three_times_internal(tree).map_err(MyAwesomeError::from)
}
但是,如果没有附加功能,如何实现它呢?
[这是一个示例,当您在函数式编程中使用Option
之类的各种包装时。在函数式编程中,有所谓的“纯”函数,它们不更改某些状态(全局变量,输出参数)而仅依赖于输入参数,并且仅将其结果作为返回值返回而没有任何副作用。它使程序更具可预测性和安全性,但带来了一些不便。