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//! This example show the pattern "Strings and custom fail type" described in the book
extern crate core;
extern crate failure;
use core::fmt::{self, Display};
use failure::{Backtrace, Context, Fail, ResultExt};
fn main() {
let err = err1().unwrap_err();
// Print the error itself
println!("error: {}", err);
// Print the chain of errors that caused it
for cause in Fail::iter_causes(&err) {
println!("caused by: {}", cause);
}
}
fn err1() -> Result<(), MyError> {
// The context can be a String
Ok(err2().context("err1".to_string())?)
}
fn err2() -> Result<(), MyError> {
// The context can be a &'static str
Ok(err3().context("err2")?)
}
fn err3() -> Result<(), MyError> {
Ok(Err(MyError::from("err3"))?)
}
#[derive(Debug)]
pub struct MyError {
inner: Context<String>,
}
impl Fail for MyError {
fn cause(&self) -> Option<&Fail> {
self.inner.cause()
}
fn backtrace(&self) -> Option<&Backtrace> {
self.inner.backtrace()
}
}
impl Display for MyError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
Display::fmt(&self.inner, f)
}
}
// Allows writing `MyError::from("oops"))?`
impl From<&'static str> for MyError {
fn from(msg: &'static str) -> MyError {
MyError {
inner: Context::new(msg.into()),
}
}
}
// Allows adding more context via a String
impl From<Context<String>> for MyError {
fn from(inner: Context<String>) -> MyError {
MyError { inner }
}
}
// Allows adding more context via a &str
impl From<Context<&'static str>> for MyError {
fn from(inner: Context<&'static str>) -> MyError {
MyError {
inner: inner.map(|s| s.to_string()),
}
}
}
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