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//! A string-based error type. //! //! # Introduction //! //! This crate provides a string-based error type, `StrError`, that //! implements `std::error::Error`. `StrError`s behave much like //! `String`s, except they may contain another error boxed inside //! them, known as the "source" or "cause". Since the source can have //! a source itself, sources form chains of errors, each error adding //! context to the preceeding one. //! //! When a `StrError` is returned from `main`, its `Debug` //! implementation causes the output of a CLI application to look like //! this //! //! ```text //! Error: ... //! Caused by: ... //! Caused by: ... //! ... //! ``` //! //! Each "Caused by:" line corresponds to a boxed error in the chain //! of sources. //! //! # The prelude //! //! This crate has a prelude to bring in all the things you need at //! once. //! //! ``` //! use strerror::prelude::*; //! ``` //! //! Because some of the crate's functionality is contained in //! extension traits, this is quite useful. The examples below all //! assume the prelude is used. //! //! # Creating `StrError`s //! //! As with `String`s, there are quite a few ways to create a //! `StrError`. Some have a `String` equivalent so we present these //! alongside each other. //! //! ``` //! # use strerror::prelude::*; //! // String // StrError //! let str1 = "Error!".to_string(); let err1 = "Error!".to_error(); //! let str2 = String::from("Error!"); let err2 = StrError::from("Error!"); //! let str3: String = "Error!".into(); let err3: StrError = "Error!".into(); //! let str4 = format!("Error! #{}", 1); let err4 = eformat!("Error! #{}", 1); //! ``` //! //! The above lines all create `StrError`s without a "source" or //! "cause". Next we show two equivalent ways to create a `StrError` //! containing a source. The `StrError` takes ownership of the source //! which may or may not be another `StrError`. //! //! ``` //! # use strerror::prelude::*; //! use std::io::Error as IoError; //! //! let source1 = IoError::from_raw_os_error(5); //! let err1 = StrError::from_error(source1, "I/O error occurred"); //! //! let source2 = IoError::from_raw_os_error(5); //! let err2 = source2.chain("I/O error occurred"); //! ``` //! //! The advantage of the second way using the `chain` method comes //! from the fact that, in chaining methods together, an error chain //! itself can be created. //! //! ``` //! # use strerror::prelude::*; //! fn main() -> Result<(), StrError> { //! # || -> Result<(), StrError> { //! let err = "Base error".to_error() //! .chain("Higher level error") //! .chain("Application error"); //! Err(err) //! # }().unwrap_err(); Ok(()) //! } //! ``` //! //! gives output //! //! ```text //! Error: Application error //! Caused by: Higher level error //! Caused by: Base error //! ``` //! //! # Working with `Result`s and `Option`s //! //! While the `chain` method adds context to error types directly, we //! can do a similar thing with the `Err` value within a `Result` with //! the `chain_err` method. //! //! ``` //! # use strerror::prelude::*; //! use std::fs::File; //! //! fn main() -> Result<(), StrError> { //! # || -> Result<(), StrError> { //! let file = "missing-file"; //! let _ = File::open(file) // returns a Result //! .chain_err(format!("Failed to open file {}", file))?; // main exits here //! Ok(()) //! # }().unwrap_err(); Ok(()) //! } //! ``` //! //! gives output //! //! ```text //! Error: Failed to open file missing-file //! Caused by: No such file or directory (os error 2) //! ``` //! //! With `Option`s no special method is needed to transform one into a //! `Result` with a `StrError`. We can just use `ok_or` and //! `ok_or_else` with a newly created one. //! //! However it is often sufficient to pass just a `&str` or //! `String`. This gives a `Result` containing a `&str` or `String` //! for its `Err` value, but when used with the `?` operator it gets //! converted to a `StrError`. This works because `StrError` //! implements `From<&str>` and `From<String>`. //! //! ``` //! # use strerror::prelude::*; //! use std::env; //! //! fn main() -> Result<(), StrError> { //! # || -> Result<(), StrError> { //! let _ = env::var_os("MISSING_VAR") // returns an Option //! .ok_or("Environment variable MISSING_VAR not found")?; // main exits here //! Ok(()) //! # }().unwrap_err(); Ok(()) //! } //! ``` //! //! gives //! //! ```text //! Error: Environment variable MISSING_VAR not found //! ``` //! //! # `From` conversions for other error types //! //! `From` conversions are implemented for most of the standard //! library error types, so you can can return `Results` containing //! them directly from a function that returns a `Result` containing a //! `StrError` using the `?` operator. //! //! ``` //! # use strerror::prelude::*; //! use std::fs::File; //! //! fn main() -> Result<(), StrError> { //! # || -> Result<(), StrError> { //! let file = "missing-file"; //! let _ = File::open(file)?; // main exits here //! Ok(()) //! # }().unwrap_err(); Ok(()) //! } //! ``` //! //! gives output //! //! ```text //! Error: std::io::Error //! Caused by: No such file or directory (os error 2) //! ``` //! //! As mentioned when discussing `Option`s, `From` conversions are //! also implemented for `&str` and `String`. //! //! However for other error types, if you wish to use the `?` //! operator, you will first need to call the `chain_err` method to //! convert the `Result` into one containing a `StrError`. Of course //! you may choose to return a `Result` containing a //! `Box<dyn std::error::Error>` from your function instead, for which //! `?` will work for all error types. //! //! # `Deref` //! //! `StrError`s deref to a `String`, so you can use many of the usual //! `String` methods. //! //! ``` //! # use strerror::prelude::*; //! fn main() -> Result<(), StrError> { //! # || -> Result<(), StrError> { //! let mut err = "This is".to_error(); //! *err += " an error"; //! err.push_str(" message"); //! Err(err) //! # }().unwrap_err(); Ok(()) //! } //! ``` //! //! gives output //! //! ```text //! Error: This is an error message //! ``` //! //! # Iterating through the source chain //! //! A reference to a `StrError` can be iterated over to examine its //! chain of boxed sources. //! //! ``` //! # use strerror::prelude::*; //! use std::io::Error as IoError; //! //! fn main() -> Result<(), StrError> { //! let err = IoError::from_raw_os_error(5) //! .chain("Failure reading disk") //! .chain("Application error"); //! for e in &err { //! println!( //! "Error: {:31} Is StrError?: {}", //! e, //! e.downcast_ref::<StrError>().is_some() //! ); //! } //! Ok(()) //! } //! ``` //! //! gives output //! //! ```text //! Error: Application error Is StrError?: true //! Error: Failure reading disk Is StrError?: true //! Error: Input/output error (os error 5) Is StrError?: false //! ``` use std::borrow::Cow; use std::error::Error; use std::fmt::Error as FmtError; use std::fmt::{Debug, Display, Formatter}; use std::ops::{Deref, DerefMut}; /// Reexports of `eformat`, `ErrorChainExt`, `ResultChainErrExt`, /// `StrError` and `StringToErrorExt`. pub mod prelude { pub use super::{ eformat, ErrorChainExt, ResultChainErrExt, StrError, StringToErrorExt, }; } /// A string-based error type implementing `std::error::Error`. /// /// `From` conversions to `StrError` are implemented for most standard /// library error types, and for `String`s and `&str`s. `Deref` /// converts to a `String`. /// /// See crate level documentation for usage examples. pub struct StrError { description: String, source: Option<Box<dyn Error + Send + Sync>>, } impl StrError { /// Convert an error of any type to a `StrError`, adding context /// and boxing it. pub fn from_error<T, E>(err: E, context: T) -> Self where T: Into<String>, E: Into<Box<dyn Error + Send + Sync>>, { StrError { description: context.into(), source: Some(err.into()) } } /// Create an iterator over a `StrError` and it's sources. When /// using the iterator, the first item retrieved is a reference to /// the `StrError` itself (as a trait object). This is then /// followed by the chain of sources. You can also use /// `&StrError`'s implementation of `IntoIterator` to obtain an /// iterator. pub fn iter(&self) -> Iter<'_> { Iter { next: Some(self) } } } impl Error for StrError { /// Return the lower-level source of this `StrError`, if any. fn source(&self) -> Option<&(dyn Error + 'static)> { self.source.as_ref().map(|e| &**e as &(dyn Error + 'static)) } } impl Display for StrError { fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), FmtError> { Display::fmt(&**self, f) } } impl Debug for StrError { fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), FmtError> { write!(f, "{}", self)?; for e in self.iter().skip(1) { write!(f, "\nCaused by: {}", e)?; } Ok(()) } } /// An iterator producing a reference to a `StrError` (as a trait /// object), followed by its chain of sources. pub struct Iter<'a> { next: Option<&'a (dyn Error + 'static)>, } impl<'a> Iterator for Iter<'a> { type Item = &'a (dyn Error + 'static); fn next(&mut self) -> Option<Self::Item> { match &self.next { None => None, Some(e) => { let next = self.next; self.next = e.source(); next } } } } impl<'a> IntoIterator for &'a StrError { type Item = &'a (dyn Error + 'static); type IntoIter = Iter<'a>; fn into_iter(self) -> Iter<'a> { self.iter() } } /// Trait providing `chain` for converting an error of any type to a /// `StrError`. /// /// See crate level documentation for usage examples. pub trait ErrorChainExt<T> { /// Convert an error of any type to a `StrError`, adding context. fn chain(self, context: T) -> StrError; } impl<T, E> ErrorChainExt<T> for E where T: Into<String>, E: Into<Box<dyn Error + Send + Sync>>, { fn chain(self, context: T) -> StrError { StrError::from_error(self.into(), context.into()) } } /// Trait providing `chain_err` for mapping the `Err` value within a /// `Result` to a `StrError`. /// /// See crate level documentation for usage examples. pub trait ResultChainErrExt<T, U> { /// Map the `Err` value within a `Result` to a `StrError`, adding /// context. fn chain_err(self, context: T) -> Result<U, StrError>; } impl<T, U, E> ResultChainErrExt<T, U> for Result<U, E> where T: Into<String>, E: Into<Box<dyn Error + Send + Sync>>, { fn chain_err(self, context: T) -> Result<U, StrError> { self.map_err(|e| StrError::from_error(e.into(), context.into())) } } /// Trait providing `to_error` for converting a `String` or `&str` to /// a `StrError`. /// /// See crate level documentation for usage examples. pub trait StringToErrorExt { /// Convert a `String` or `&str` to a `StrError`. fn to_error(self) -> StrError; } impl<T> StringToErrorExt for T where T: Into<String>, { fn to_error(self) -> StrError { StrError::from(self.into()) } } /// A macro for creating a `StrError` using interpolation of runtime /// expressions (like `format!`). /// /// `eformat!` is an extremely simple macro meant to save some /// keystrokes when creating `StrError`s. The name was chosen to /// mirror that of `eprint!` in the standard library, which is an /// "error" version of `print!`. /// /// Call `eformat!` as you would call `format!`, but you will get a /// `StrError` instead of a `String`. #[macro_export] macro_rules! eformat { ($($arg:tt)*) => { StrError::from(format!($($arg)*)) } } // Deref conversion for StrError. impl Deref for StrError { type Target = String; fn deref(&self) -> &String { &self.description } } impl DerefMut for StrError { fn deref_mut(&mut self) -> &mut String { &mut self.description } } // From conversions for Strings, &strs etc impl From<Box<str>> for StrError { fn from(s: Box<str>) -> Self { StrError { description: s.into_string(), source: None } } } impl<'a> From<Cow<'a, str>> for StrError { fn from(s: Cow<'a, str>) -> Self { StrError { description: s.into_owned(), source: None } } } impl From<&str> for StrError { fn from(s: &str) -> Self { StrError { description: s.to_owned(), source: None } } } impl From<String> for StrError { fn from(s: String) -> Self { StrError { description: s, source: None } } } impl From<&String> for StrError { fn from(s: &String) -> Self { StrError { description: s.clone(), source: None } } } // From conversions for stdlib errors. macro_rules! impl_from { ($from:ty) => { impl From<$from> for StrError { fn from(err: $from) -> Self { StrError::from_error(err, stringify!($from)) } } }; ($from:ty, $param:ident $(, $bound:ident)* ) => { impl<$param> From<$from> for StrError where $param: Send + Sync + 'static $(+ $bound)*, { fn from(err: $from) -> Self { StrError::from_error(err, stringify!($from)) } } }; } impl_from!(std::alloc::LayoutErr); impl_from!(std::array::TryFromSliceError); impl_from!(std::boxed::Box<T>, T, Error); impl_from!(std::cell::BorrowError); impl_from!(std::cell::BorrowMutError); impl_from!(std::char::CharTryFromError); impl_from!(std::char::DecodeUtf16Error); impl_from!(std::char::ParseCharError); impl_from!(std::env::JoinPathsError); impl_from!(std::env::VarError); impl_from!(std::ffi::FromBytesWithNulError); impl_from!(std::ffi::IntoStringError); impl_from!(std::ffi::NulError); impl_from!(std::fmt::Error); impl_from!(std::io::Error); impl_from!(std::io::IntoInnerError<T>, T, Debug); impl_from!(std::net::AddrParseError); impl_from!(std::num::ParseFloatError); impl_from!(std::num::ParseIntError); impl_from!(std::num::TryFromIntError); impl_from!(std::path::StripPrefixError); impl_from!(std::str::ParseBoolError); impl_from!(std::str::Utf8Error); impl_from!(std::string::FromUtf16Error); impl_from!(std::string::FromUtf8Error); impl_from!(std::string::ParseError); impl_from!(std::sync::PoisonError<T>, T); impl_from!(std::sync::TryLockError<T>, T); impl_from!(std::sync::mpsc::RecvError); impl_from!(std::sync::mpsc::RecvTimeoutError); impl_from!(std::sync::mpsc::SendError<T>, T); impl_from!(std::sync::mpsc::TryRecvError); impl_from!(std::sync::mpsc::TrySendError<T>, T); impl_from!(std::time::SystemTimeError);