1Anyhow ¯\\\_(°ペ)\_/¯
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 1.39+*
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 Rust &ge; 1.65, a backtrace is captured and printed with the error if
79  the underlying error type does not already provide its own. In order to see
80  backtraces, they must be enabled through the environment variables described
81  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  [`std::backtrace`]: https://doc.rust-lang.org/std/backtrace/index.html#environment-variables
90
91- Anyhow works with any error type that has an impl of `std::error::Error`,
92  including ones defined in your crate. We do not bundle a `derive(Error)` macro
93  but you can write the impls yourself or use a standalone macro like
94  [thiserror].
95
96  ```rust
97  use thiserror::Error;
98
99  #[derive(Error, Debug)]
100  pub enum FormatError {
101      #[error("Invalid header (expected {expected:?}, got {found:?})")]
102      InvalidHeader {
103          expected: String,
104          found: String,
105      },
106      #[error("Missing attribute: {0}")]
107      MissingAttribute(String),
108  }
109  ```
110
111- One-off error messages can be constructed using the `anyhow!` macro, which
112  supports string interpolation and produces an `anyhow::Error`.
113
114  ```rust
115  return Err(anyhow!("Missing attribute: {}", missing));
116  ```
117
118  A `bail!` macro is provided as a shorthand for the same early return.
119
120  ```rust
121  bail!("Missing attribute: {}", missing);
122  ```
123
124<br>
125
126## No-std support
127
128In no_std mode, the same API is almost all available and works the same way. To
129depend on Anyhow in no_std mode, disable our default enabled "std" feature in
130Cargo.toml. A global allocator is required.
131
132```toml
133[dependencies]
134anyhow = { version = "1.0", default-features = false }
135```
136
137Since the `?`-based error conversions would normally rely on the
138`std::error::Error` trait which is only available through std, no_std mode will
139require an explicit `.map_err(Error::msg)` when working with a non-Anyhow error
140type inside a function that returns Anyhow's error type.
141
142<br>
143
144## Comparison to failure
145
146The `anyhow::Error` type works something like `failure::Error`, but unlike
147failure ours is built around the standard library's `std::error::Error` trait
148rather than a separate trait `failure::Fail`. The standard library has adopted
149the necessary improvements for this to be possible as part of [RFC 2504].
150
151[RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md
152
153<br>
154
155## Comparison to thiserror
156
157Use Anyhow if you don't care what error type your functions return, you just
158want it to be easy. This is common in application code. Use [thiserror] if you
159are a library that wants to design your own dedicated error type(s) so that on
160failures the caller gets exactly the information that you choose.
161
162[thiserror]: https://github.com/dtolnay/thiserror
163
164<br>
165
166#### License
167
168<sup>
169Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
1702.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
171</sup>
172
173<br>
174
175<sub>
176Unless you explicitly state otherwise, any contribution intentionally submitted
177for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
178be dual licensed as above, without any additional terms or conditions.
179</sub>
180