1Anyhow ¯\\\_(ツ)\_/¯
2=========================
3
4[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/anyhow-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/anyhow)
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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 1.34+*
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, a backtrace is captured and printed with the
79  error if the underlying error type does not already provide its own. In order
80  to see backtraces, they must be enabled through the environment variables
81  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<br>
122
123## No-std support
124
125In no_std mode, the same API is almost all available and works the same way. To
126depend on Anyhow in no_std mode, disable our default enabled "std" feature in
127Cargo.toml. A global allocator is required.
128
129```toml
130[dependencies]
131anyhow = { version = "1.0", default-features = false }
132```
133
134Since the `?`-based error conversions would normally rely on the
135`std::error::Error` trait which is only available through std, no_std mode will
136require an explicit `.map_err(Error::msg)` when working with a non-Anyhow error
137type inside a function that returns Anyhow's error type.
138
139<br>
140
141## Comparison to failure
142
143The `anyhow::Error` type works something like `failure::Error`, but unlike
144failure ours is built around the standard library's `std::error::Error` trait
145rather than a separate trait `failure::Fail`. The standard library has adopted
146the necessary improvements for this to be possible as part of [RFC 2504].
147
148[RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md
149
150<br>
151
152## Comparison to thiserror
153
154Use Anyhow if you don't care what error type your functions return, you just
155want it to be easy. This is common in application code. Use [thiserror] if you
156are a library that wants to design your own dedicated error type(s) so that on
157failures the caller gets exactly the information that you choose.
158
159[thiserror]: https://github.com/dtolnay/thiserror
160
161<br>
162
163#### License
164
165<sup>
166Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
1672.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
168</sup>
169
170<br>
171
172<sub>
173Unless you explicitly state otherwise, any contribution intentionally submitted
174for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
175be dual licensed as above, without any additional terms or conditions.
176</sub>
177