1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Traits for working with Errors.
13 //! # The `Error` trait
15 //! `Error` is a trait representing the basic expectations for error values,
16 //! i.e. values of type `E` in `Result<T, E>`. At a minimum, errors must provide
17 //! a description, but they may optionally provide additional detail (via
18 //! `Display`) and cause chain information:
21 //! use std::fmt::Display;
23 //! trait Error: Display {
24 //! fn description(&self) -> &str;
26 //! fn cause(&self) -> Option<&Error> { None }
30 //! The `cause` method is generally used when errors cross "abstraction
31 //! boundaries", i.e. when a one module must report an error that is "caused"
32 //! by an error from a lower-level module. This setup makes it possible for the
33 //! high-level module to provide its own errors that do not commit to any
34 //! particular implementation, but also reveal some of its implementation for
35 //! debugging via `cause` chains.
37 //! # The `FromError` trait
39 //! `FromError` is a simple trait that expresses conversions between different
40 //! error types. To provide maximum flexibility, it does not require either of
41 //! the types to actually implement the `Error` trait, although this will be the
44 //! The main use of this trait is in the `try!` macro, which uses it to
45 //! automatically convert a given error to the error specified in a function's
51 //! use std::error::FromError;
52 //! use std::io::{File, IoError};
53 //! use std::os::{MemoryMap, MapError};
54 //! use std::path::Path;
61 //! impl FromError<IoError> for MyError {
62 //! fn from_error(err: IoError) -> MyError {
67 //! impl FromError<MapError> for MyError {
68 //! fn from_error(err: MapError) -> MyError {
73 //! #[allow(unused_variables)]
74 //! fn open_and_map() -> Result<(), MyError> {
75 //! let f = try!(File::open(&Path::new("foo.txt")));
76 //! let m = try!(MemoryMap::new(0, &[]));
77 //! // do something interesting here...
87 /// Base functionality for all errors in Rust.
88 #[unstable = "the exact API of this trait may change"]
89 pub trait Error: Display {
90 /// A short description of the error; usually a static string.
91 fn description(&self) -> &str;
93 /// The lower-level cause of this error, if any.
94 fn cause(&self) -> Option<&Error> { None }
97 /// A trait for types that can be converted from a given error type `E`.
99 pub trait FromError<E> {
100 /// Perform the conversion.
101 fn from_error(err: E) -> Self;
104 // Any type is convertable from itself
106 impl<E> FromError<E> for E {
107 fn from_error(err: E) -> E {