README.md
1Anyhow ¯\\\_(°ペ)\_/¯
2==========================
3
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8
9This library provides [`anyhow::Error`][Error], a trait object based error type
10for easy idiomatic error handling in Rust applications.
11
12[Error]: https://docs.rs/anyhow/1.0/anyhow/struct.Error.html
13
14```toml
15[dependencies]
16anyhow = "1.0"
17```
18
19*Compiler support: requires rustc 1.38+*
20
21<br>
22
23## Details
24
25- Use `Result<T, anyhow::Error>`, or equivalently `anyhow::Result<T>`, as the
26 return type of any fallible function.
27
28 Within the function, use `?` to easily propagate any error that implements the
29 `std::error::Error` trait.
30
31 ```rust
32 use anyhow::Result;
33
34 fn get_cluster_info() -> Result<ClusterMap> {
35 let config = std::fs::read_to_string("cluster.json")?;
36 let map: ClusterMap = serde_json::from_str(&config)?;
37 Ok(map)
38 }
39 ```
40
41- Attach context to help the person troubleshooting the error understand where
42 things went wrong. A low-level error like "No such file or directory" can be
43 annoying to debug without more context about what higher level step the
44 application was in the middle of.
45
46 ```rust
47 use anyhow::{Context, Result};
48
49 fn main() -> Result<()> {
50 ...
51 it.detach().context("Failed to detach the important thing")?;
52
53 let content = std::fs::read(path)
54 .with_context(|| format!("Failed to read instrs from {}", path))?;
55 ...
56 }
57 ```
58
59 ```console
60 Error: Failed to read instrs from ./path/to/instrs.json
61
62 Caused by:
63 No such file or directory (os error 2)
64 ```
65
66- Downcasting is supported and can be by value, by shared reference, or by
67 mutable reference as needed.
68
69 ```rust
70 // If the error was caused by redaction, then return a
71 // tombstone instead of the content.
72 match root_cause.downcast_ref::<DataStoreError>() {
73 Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)),
74 None => Err(error),
75 }
76 ```
77
78- If using the nightly channel, or stable with `features = ["backtrace"]`, a
79 backtrace is captured and printed with the error if the underlying error type
80 does not already provide its own. In order to see backtraces, they must be
81 enabled through the environment variables described in [`std::backtrace`]:
82
83 - If you want panics and errors to both have backtraces, set
84 `RUST_BACKTRACE=1`;
85 - If you want only errors to have backtraces, set `RUST_LIB_BACKTRACE=1`;
86 - If you want only panics to have backtraces, set `RUST_BACKTRACE=1` and
87 `RUST_LIB_BACKTRACE=0`.
88
89 The tracking issue for this feature is [rust-lang/rust#53487].
90
91 [`std::backtrace`]: https://doc.rust-lang.org/std/backtrace/index.html#environment-variables
92 [rust-lang/rust#53487]: https://github.com/rust-lang/rust/issues/53487
93
94- Anyhow works with any error type that has an impl of `std::error::Error`,
95 including ones defined in your crate. We do not bundle a `derive(Error)` macro
96 but you can write the impls yourself or use a standalone macro like
97 [thiserror].
98
99 ```rust
100 use thiserror::Error;
101
102 #[derive(Error, Debug)]
103 pub enum FormatError {
104 #[error("Invalid header (expected {expected:?}, got {found:?})")]
105 InvalidHeader {
106 expected: String,
107 found: String,
108 },
109 #[error("Missing attribute: {0}")]
110 MissingAttribute(String),
111 }
112 ```
113
114- One-off error messages can be constructed using the `anyhow!` macro, which
115 supports string interpolation and produces an `anyhow::Error`.
116
117 ```rust
118 return Err(anyhow!("Missing attribute: {}", missing));
119 ```
120
121 A `bail!` macro is provided as a shorthand for the same early return.
122
123 ```rust
124 bail!("Missing attribute: {}", missing);
125 ```
126
127<br>
128
129## No-std support
130
131In no_std mode, the same API is almost all available and works the same way. To
132depend on Anyhow in no_std mode, disable our default enabled "std" feature in
133Cargo.toml. A global allocator is required.
134
135```toml
136[dependencies]
137anyhow = { version = "1.0", default-features = false }
138```
139
140Since the `?`-based error conversions would normally rely on the
141`std::error::Error` trait which is only available through std, no_std mode will
142require an explicit `.map_err(Error::msg)` when working with a non-Anyhow error
143type inside a function that returns Anyhow's error type.
144
145<br>
146
147## Comparison to failure
148
149The `anyhow::Error` type works something like `failure::Error`, but unlike
150failure ours is built around the standard library's `std::error::Error` trait
151rather than a separate trait `failure::Fail`. The standard library has adopted
152the necessary improvements for this to be possible as part of [RFC 2504].
153
154[RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md
155
156<br>
157
158## Comparison to thiserror
159
160Use Anyhow if you don't care what error type your functions return, you just
161want it to be easy. This is common in application code. Use [thiserror] if you
162are a library that wants to design your own dedicated error type(s) so that on
163failures the caller gets exactly the information that you choose.
164
165[thiserror]: https://github.com/dtolnay/thiserror
166
167<br>
168
169#### License
170
171<sup>
172Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
1732.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
174</sup>
175
176<br>
177
178<sub>
179Unless you explicitly state otherwise, any contribution intentionally submitted
180for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
181be dual licensed as above, without any additional terms or conditions.
182</sub>
183
