anyhow

https://docs.rs/anyhow/latest/anyhow
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### Rust Example: Using anyhow Context for Error Handling

Source: https://docs.rs/anyhow/latest/anyhow/trait

An example showcasing the usage of `context` and `with_context` methods from the anyhow crate to add descriptive context to errors during file operations. It demonstrates how context chains are printed and how to handle errors with context.

```Rust
use anyhow::{Context, Result};
use std::fs;
use std::path::PathBuf;

pub struct ImportantThing {
    path: PathBuf,
}

impl ImportantThing {
    pub fn detach(&mut self) -> Result<()> {...}
}

pub fn do_it(mut it: ImportantThing) -> Result<Vec<u8>> {
    it.detach().context("Failed to detach the important thing")?;

    let path = &it.path;
    let content = fs::read(path)
        .with_context(|| format!("Failed to read instrs from {}", path.display()))?;

    Ok(content)
}
```

--------------------------------

### Rust Example Usage of ensure Macro

Source: https://docs.rs/anyhow/latest/anyhow/macro

Demonstrates the basic usage of the 'ensure' macro in Rust with a simple condition and a literal error message.

```Rust
ensure!(user == 0, "only user 0 is allowed");
```

--------------------------------

### Rust Result Product Example

Source: https://docs.rs/anyhow/latest/anyhow/type

Demonstrates how to use the `product` method on an iterator of `Result` values. If any element in the iterator is an `Err`, the operation short-circuits and returns that `Err`. Otherwise, it returns the product of all `Ok` values.

```Rust
let nums = vec!["5", "10", "1", "2"];
let total: Result<usize, _> = nums.iter().map(|w| w.parse::<usize>()).product();
assert_eq!(total, Ok(100));
let nums = vec!["5", "10", "one", "2"];
let total: Result<usize, _> = nums.iter().map(|w| w.parse::<usize>()).product();
assert!(total.is_err());
```

--------------------------------

### Rust Result iter_mut Example

Source: https://docs.rs/anyhow/latest/anyhow/type

Demonstrates mutable iteration over the contents of a Result, modifying the Ok value if present.

```Rust
let mut x: Result<u32, &str> = Ok(7);
match x.iter_mut().next() {
    Some(v) => *v = 40,
    None => {},
}
assert_eq!(x, Ok(40));

let mut x: Result<u32, &str> = Err("nothing!");
assert_eq!(x.iter_mut().next(), None);
```

--------------------------------

### Rust Result Sum Example

Source: https://docs.rs/anyhow/latest/anyhow/type

Illustrates the `sum` method for iterators of `Result` values. Similar to `product`, if an `Err` is encountered, it's returned immediately. Otherwise, the sum of all `Ok` values is computed and returned.

```Rust
let f = |&x: &i32| if x < 0 { Err("Negative element found") } else { Ok(x) };
let v = vec![1, 2];
let res: Result<i32, _> = v.iter().map(f).sum();
assert_eq!(res, Ok(3));
let v = vec![1, -2];
let res: Result<i32, _> = v.iter().map(f).sum();
assert_eq!(res, Err("Negative element found"));
```

--------------------------------

### Custom Error Rendering

Source: https://docs.rs/anyhow/latest/anyhow/struct

Provides an example of how to manually render an error and its cause chain using `anyhow::Context` and iterating through the error chain.

```Rust
use anyhow::{Context, Result};

fn main() {
    if let Err(err) = try_main() {
        eprintln!("ERROR: {}", err);
        err.chain().skip(1).for_each(|cause| eprintln!("because: {}", cause));
        std::process::exit(1);
    }
}

fn try_main() -> Result<()> {
    ...
}
```

--------------------------------

### Main Function with anyhow::Result for Error Handling

Source: https://docs.rs/anyhow/latest/anyhow/type

An example of a `main` function that returns `anyhow::Result<()>`. This pattern allows for convenient error propagation using the `?` operator for operations that return `Result`, such as file reading and JSON parsing. Failures are automatically printed with context and a backtrace if captured.

```Rust
use anyhow::Result;

fn main() -> Result<()> {
    let config = std::fs::read_to_string("cluster.json")?;
    let map: ClusterMap = serde_json::from_str(&config)?;
    println!("cluster info: {:#?}", map);
    Ok(())
}
```

--------------------------------

### Rust Result unwrap Method

Source: https://docs.rs/anyhow/latest/anyhow/type

Demonstrates the `unwrap` method to get the Ok value from a Result. This method panics if the Result is an Err, and its use is generally discouraged in favor of explicit error handling.

```Rust
let x: Result<u32, &str> = Ok(2);
assert_eq!(x.unwrap(), 2);
```

```Rust
let x: Result<u32, &str> = Err("emergency failure");
x.unwrap(); // panics with `emergency failure`
```

--------------------------------

### Rust Example: Downcasting with anyhow Context

Source: https://docs.rs/anyhow/latest/anyhow/trait

Illustrates how to use anyhow's context feature while maintaining the ability to downcast errors. It shows two scenarios: attaching insignificant context to downcastable errors and attaching downcastable context to insignificant errors.

```Rust
use anyhow::{Context, Result};

fn do_it() -> Result<()> {
    helper().context("Failed to complete the work")?;
    ...
}

fn main() {
    let err = do_it().unwrap_err();
    if let Some(e) = err.downcast_ref::<SuspiciousError>() {
        // If helper() returned SuspiciousError, this downcast will
        // correctly succeed even with the context in between.
    }
}
```

```Rust
use anyhow::{Context, Result};

fn do_it() -> Result<()> {
    helper().context(HelperFailed)?;
    ...
}

fn main() {
    let err = do_it().unwrap_err();
    if let Some(e) = err.downcast_ref::<HelperFailed>() {
        // If helper failed, this downcast will succeed because
        // HelperFailed is the context that has been attached to
        // that error.
    }
}
```

--------------------------------

### Rust Example Usage of ensure with Custom Error

Source: https://docs.rs/anyhow/latest/anyhow/macro

Shows how to use the 'ensure' macro in Rust with a custom error enum, returning an instance of the enum if the condition fails.

```Rust
#[derive(Error, Debug)]
enum ScienceError {
    #[error("recursion limit exceeded")]
    RecursionLimitExceeded,
    ...
}

ensure!(depth <= MAX_DEPTH, ScienceError::RecursionLimitExceeded);
```

--------------------------------

### Example Usage of anyhow::Result

Source: https://docs.rs/anyhow/latest/anyhow/type

Demonstrates how to use the `anyhow::Result` type alias in function signatures. It shows two common patterns: `Result<T>` which defaults to `anyhow::Error`, and `Result<T, OtherError>` for functions that might return a different specific error type.

```Rust
use anyhow::Result;

fn demo1() -> Result<T> {...}
           // ^ equivalent to std::result::Result<T, anyhow::Error>

fn demo2() -> Result<T, OtherError> {...}
           // ^ equivalent to std::result::Result<T, OtherError>
```

--------------------------------

### Rust: Get Value or Default with `unwrap_or`

Source: https://docs.rs/anyhow/latest/anyhow/type

The `unwrap_or` function returns the contained `Ok` value or a provided default if the `Result` is `Err`. Arguments are eagerly evaluated.

```Rust
let default = 2;
let x: Result<u32, &str> = Ok(9);
assert_eq!(x.unwrap_or(default), 9);

let x: Result<u32, &str> = Err("error");
assert_eq!(x.unwrap_or(default), default);
```

--------------------------------

### Rust Type Inference Error with anyhow::Result::Ok

Source: https://docs.rs/anyhow/latest/anyhow/fn

Illustrates a common type inference error (`E0282`) that can occur when using `anyhow::Result::Ok` without explicit type annotations. The example shows how the compiler cannot infer the error type `E` for `std::result::Result<i32, E>`.

```Rust
error[E0282]: type annotations needed for `std::result::Result<i32, E>`
  --> src/main.rs:11:13
   | 
11 |     let _ = anyhow::Result::Ok(1);
   |         -   ^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `E` declared on the enum `Result`
   |         |
   |         consider giving this pattern the explicit type `std::result::Result<i32, E>`, where the type parameter `E` is specified

```

--------------------------------

### Using the anyhow Macro in Rust

Source: https://docs.rs/anyhow/latest/anyhow/all

Demonstrates the usage of the 'anyhow' macro for creating errors in Rust. This macro is part of the anyhow crate and simplifies error handling by providing context.

```Rust
use anyhow::anyhow;

fn main() {
    let err = anyhow!("Something went wrong");
    println!("{}", err);
}
```

--------------------------------

### Rust: Get Type ID

Source: https://docs.rs/anyhow/latest/anyhow/struct

Retrieves the `TypeId` of the current type. This is part of the `Any` trait and is used for runtime type identification, often in conjunction with downcasting.

```rust
fn type_id(&self) -> TypeId
```

--------------------------------

### Rust Result expect Method

Source: https://docs.rs/anyhow/latest/anyhow/type

Shows how to use the `expect` method to retrieve the Ok value from a Result, panicking with a custom message if it's an Err. It's recommended to use pattern matching instead.

```Rust
let x: Result<u32, &str> = Err("emergency failure");
x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
```

```Rust
let path = std::env::var("IMPORTANT_PATH")
    .expect("env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`");
```

--------------------------------

### Rust Ok Function for anyhow::Result

Source: https://docs.rs/anyhow/latest/anyhow/fn

Demonstrates the usage of the `anyhow::Result::Ok` function, which simplifies the creation of `anyhow::Result` instances. It's particularly useful when type inference might otherwise fail to deduce the error type.

```Rust
pub fn Ok<T>(value: T) -> Result<T>
```

--------------------------------

### Rust Trait Context Implementation

Source: https://docs.rs/anyhow/latest/anyhow/trait

Demonstrates the `Context` trait in Rust's anyhow crate, which provides methods to wrap errors with additional context. It includes the `context` and `with_context` methods.

```Rust
pub trait Context<T, E>: Sealed {
    // Required methods
    fn context<C>(self, context: C) -> Result<T, Error>
       where C: Display + Send + Sync + 'static;
with_context<C, F>(self, f: F) -> Result<T, Error>
       where C: Display + Send + Sync + 'static,
             F: FnOnce() -> C;
}
```

--------------------------------

### Get Underlying IO Error Kind

Source: https://docs.rs/anyhow/latest/anyhow/struct

Iterates through the error chain to find the first underlying io::Error and returns its kind. This is useful for specific error handling based on IO operations.

```Rust
use anyhow::Error;
use std::io;

pub fn underlying_io_error_kind(error: &Error) -> Option<io::ErrorKind> {
    for cause in error.chain() {
        if let Some(io_error) = cause.downcast_ref::<io::Error>() {
            return Some(io_error.kind());
        }
    }
    None
}
```

--------------------------------

### Rust: No-std Configuration for anyhow

Source: https://docs.rs/anyhow/latest/anyhow/index

Illustrates how to configure the `anyhow` crate for `no_std` environments by disabling the default "std" feature in `Cargo.toml` and mentions the requirement for a global allocator.

```toml
[dependencies]
anyhow = { version = "1.0", default-features = false }
```

--------------------------------

### Result::transpose Method for Option<T>

Source: https://docs.rs/anyhow/latest/anyhow/type

Explains the `transpose` method for `Result<Option<T>, E>`, which converts a `Result` containing an `Option` into an `Option` containing a `Result`. `Ok(None)` becomes `None`, while `Ok(Some(v))` and `Err(e)` become `Some(Ok(v))` and `Some(Err(e))` respectively.

```Rust
#[derive(Debug, Eq, PartialEq)]
struct SomeErr;

let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
assert_eq!(x.transpose(), y);
```

--------------------------------

### Using the bail Macro in Rust

Source: https://docs.rs/anyhow/latest/anyhow/all

Illustrates how to use the 'bail' macro to return an error immediately from a function in Rust. This is useful for early exit conditions.

```Rust
use anyhow::bail;

fn might_fail(condition: bool) -> anyhow::Result<()> {
    if !condition {
        bail!("Operation failed due to condition");
    }
    Ok(())
}
```

--------------------------------

### Rust Result into_ok Method

Source: https://docs.rs/anyhow/latest/anyhow/type

Introduces the nightly-only `into_ok` method, which safely returns the Ok value from a Result without panicking. It's a compile-time safeguard against Results that can never be Err.

```Rust
fn only_good_news() -> Result<String, !> {
    Ok("this is fine".into())
}

let s: String = only_good_news().into_ok();
println!("{s}");
```

--------------------------------

### Rust: Construct Result from Residual

Source: https://docs.rs/anyhow/latest/anyhow/type

Provides experimental nightly-only APIs to construct a Result from compatible Residual types, specifically `Result<Infallible, E>` and `Yeet<E>`.

```Rust
impl<T, E, F>
where F: From<E>,
{
    fn from_residual(residual: Result<Infallible, E>) -> Result<T, F>;
}

impl<T, E, F>
where F: From<E>,
{
    fn from_residual(_: Yeet<E>) -> Result<T, F>;
}
```

--------------------------------

### Using the ensure Macro in Rust

Source: https://docs.rs/anyhow/latest/anyhow/all

Shows the usage of the 'ensure' macro to check a condition and return an error if it's not met in Rust. This is commonly used for input validation.

```Rust
use anyhow::ensure;

fn process_data(value: i32) -> anyhow::Result<()>
{
    ensure!(value > 0, "Value must be positive, got {}", value);
    // Process the data
    Ok(())
}
```

--------------------------------

### Using anyhow::Result in Rust

Source: https://docs.rs/anyhow/latest/anyhow/all

Demonstrates the use of the 'Result' type alias provided by the anyhow crate for simplified error handling in Rust. It combines the standard Result with anyhow's error capabilities.

```Rust
use anyhow::Result;

fn read_config() -> Result<String> {
    // Simulate reading a config file
    Ok("config_data".to_string())
}

fn main() {
    match read_config() {
        Ok(config) => println!("Config: {}", config),
        Err(e) => eprintln!("Error reading config: {}", e),
    }
}
```

--------------------------------

### Rust: Create Errors with anyhow! Macro

Source: https://docs.rs/anyhow/latest/anyhow/index

Shows the usage of the `anyhow!` macro to create ad-hoc error messages with string interpolation, which are then returned as `anyhow::Error`.

```Rust
return Err(anyhow!("Missing attribute: {}", missing));
```

--------------------------------

### Rust Result as_ref

Source: https://docs.rs/anyhow/latest/anyhow/type

Converts a reference to a Result<&T, &E> into a Result<&T, &E>. It produces a new Result containing a reference into the original, leaving the original untouched.

```Rust
let x: Result<u32, &str> = Ok(2);
assert_eq!(x.as_ref(), Ok(&2));

let x: Result<u32, &str> = Err("Error");
assert_eq!(x.as_ref(), Err(&"Error"));
```

--------------------------------

### Rust: Early Return with bail! Macro

Source: https://docs.rs/anyhow/latest/anyhow/index

Provides a shorthand using the `bail!` macro for early return of an `anyhow::Error`, simplifying the code when an error condition is met.

```Rust
bail!("Missing attribute: {}", missing);
```

--------------------------------

### Rust: Debug format for Result

Source: https://docs.rs/anyhow/latest/anyhow/type

Implements the Debug trait for Result, enabling formatted output for debugging purposes if both the Ok and Err types implement Debug.

```Rust
impl<T, E>
where T: Debug,
      E: Debug,
{
    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>;
}
```

--------------------------------

### Rust Result ok

Source: https://docs.rs/anyhow/latest/anyhow/type

Converts a Result<T, E> into an Option<T>, consuming the Result and discarding the error if present.

```Rust
let x: Result<u32, &str> = Ok(2);
assert_eq!(x.ok(), Some(2));

let x: Result<u32, &str> = Err("Nothing here");
assert_eq!(x.ok(), None);
```

--------------------------------

### Rust: Add Context to Errors with anyhow::Context

Source: https://docs.rs/anyhow/latest/anyhow/index

Shows how to add contextual information to errors using the `context()` and `with_context()` methods from `anyhow`. This helps in debugging by providing higher-level details about where an error occurred.

```Rust
use anyhow::{Context, Result};

fn main() -> Result<()> {
    ...
    it.detach().context("Failed to detach the important thing")?;

    let content = std::fs::read(path)
        .with_context(|| format!("Failed to read instrs from {}", path))?;
    ...
}
```

--------------------------------

### Rust: Propagate Errors with anyhow::Result and ?

Source: https://docs.rs/anyhow/latest/anyhow/index

Demonstrates using `anyhow::Result<T>` and the `?` operator for concise error propagation in Rust. Any error implementing `std::error::Error` can be automatically converted and returned.

```Rust
use anyhow::Result;

fn get_cluster_info() -> Result<ClusterMap> {
    let config = std::fs::read_to_string("cluster.json")?;
    let map: ClusterMap = serde_json::from_str(&config)?;
    Ok(map)
}
```

--------------------------------

### Rust: Integrate with thiserror for Custom Error Types

Source: https://docs.rs/anyhow/latest/anyhow/index

Demonstrates how to define custom error types using the `thiserror` crate and implement `std::error::Error` for them, making them compatible with `anyhow` for seamless integration.

```Rust
use thiserror::Error;

#[derive(Error, Debug)]
pub enum FormatError {
    #[error("Invalid header (expected {expected:?}, got {found:?})")]
    InvalidHeader {
        expected: String,
        found: String,
    },
    #[error("Missing attribute: {0}")]
    MissingAttribute(String),
}
```

--------------------------------

### Rust: Conditional Error Return with ensure! Macro

Source: https://docs.rs/anyhow/latest/anyhow/index

Demonstrates the `ensure!` macro for returning an error early if a specific condition is not met, improving code readability for validation checks.

```Rust
ensure!(condition, "Error message if condition is false");
```

--------------------------------

### Rust: Chain Operations with `and`

Source: https://docs.rs/anyhow/latest/anyhow/type

The `and` function returns the second `Result` if the first is `Ok`, otherwise returns the `Err` of the first. Arguments are eagerly evaluated.

```Rust
let x: Result<u32, &str> = Ok(2);
let y: Result<&str, &str> = Err("late error");
assert_eq!(x.and(y), Err("late error"));

let x: Result<u32, &str> = Err("early error");
let y: Result<&str, &str> = Ok("foo");
assert_eq!(x.and(y), Err("early error"));

let x: Result<u32, &str> = Err("not a 2");
let y: Result<&str, &str> = Err("late error");
assert_eq!(x.and(y), Err("not a 2"));

let x: Result<u32, &str> = Ok(2);
let y: Result<&str, &str> = Ok("different result type");
assert_eq!(x.and(y), Ok("different result type"));
```

--------------------------------

### Rust Result unwrap_or_default Method

Source: https://docs.rs/anyhow/latest/anyhow/type

Illustrates the `unwrap_or_default` method, which returns the Ok value or the default value for the type if the Result is an Err. This is useful for fallbacks.

```Rust
let good_year_from_input = "1909";
let bad_year_from_input = "190blarg";
let good_year = good_year_from_input.parse().unwrap_or_default();
let bad_year = bad_year_from_input.parse().unwrap_or_default();

assert_eq!(1909, good_year);
assert_eq!(0, bad_year);
```

--------------------------------

### Display Representations of Error

Source: https://docs.rs/anyhow/latest/anyhow/struct

Demonstrates how to display error information using different formatting specifiers. '{:#}' shows the cause chain, '{:?}' includes the backtrace, and '{:#?}' provides a struct-style Debug representation.

```Rust
Failed to read instrs from ./path/to/instrs.json
```

```Rust
Failed to read instrs from ./path/to/instrs.json: No such file or directory (os error 2)
```

```Rust
Error: Failed to read instrs from ./path/to/instrs.json

Caused by:
    No such file or directory (os error 2)

Stack backtrace:
   0: <E as anyhow::context::ext::StdError>::ext_context
             at /git/anyhow/src/backtrace.rs:26
   1: core::result::Result<T,E>::map_err
             at /git/rustc/src/libcore/result.rs:596
   2: anyhow::context::<impl anyhow::Context<T,E> for core::result::Result<T,E>>::with_context
             at /git/anyhow/src/context.rs:58
   3: testing::main
             at src/main.rs:5
   4: std::rt::lang_start
             at /git/rustc/src/libstd/rt.rs:61
   5: main
   6: __libc_start_main
   7: _start
```

```Rust
Error {
    context: "Failed to read instrs from ./path/to/instrs.json",
    source: Os {
        code: 2,
        kind: NotFound,
        message: "No such file or directory",
    },
}
```

--------------------------------

### Rust Function: context

Source: https://docs.rs/anyhow/latest/anyhow/trait

The `context` method from the `Context` trait in the anyhow crate. It wraps an error value with additional, immediately evaluated context.

```Rust
fn context<C>(self, context: C) -> Result<T, Error>
where C: Display + Send + Sync + 'static,
Wrap the error value with additional context.
```

--------------------------------

### Result::copied Method for &T

Source: https://docs.rs/anyhow/latest/anyhow/type

Demonstrates the `copied` method for `Result<&T, E>`, which maps a `Result` containing a reference to a value to a `Result` containing the copied value. This requires the type `T` to implement the `Copy` trait.

```Rust
let val = 12;
let x: Result<&i32, i32> = Ok(&val);
assert_eq!(x, Ok(&12));
let copied = x.copied();
assert_eq!(copied, Ok(12));
```

--------------------------------

### Add Context to Option<T> in Rust

Source: https://docs.rs/anyhow/latest/anyhow/trait

Demonstrates adding contextual error messages to an `Option<T>` using the `anyhow::Context` trait. The `context()` method is used to provide a static string as context when unwrapping the `Option`.

```Rust
use anyhow::{Context, Result};

fn maybe_get<T>() -> Option<T> {
    // Placeholder for a function that might return None
    None
}

fn demo() -> Result<()> {
    let t = maybe_get::<i32>().context("there is no T")?;
    Ok(())
}
```

--------------------------------

### Result::cloned Method for &mut T

Source: https://docs.rs/anyhow/latest/anyhow/type

Demonstrates the `cloned` method for `Result<&mut T, E>`, which maps a `Result` containing a mutable reference to a value to a `Result` containing a cloned value. This requires the type `T` to implement the `Clone` trait.

```Rust
let mut val = 12;
let x: Result<&mut i32, i32> = Ok(&mut val);
assert_eq!(x, Ok(&mut 12));
let cloned = x.cloned();
assert_eq!(cloned, Ok(12));
```

--------------------------------

### Result::cloned Method for &T

Source: https://docs.rs/anyhow/latest/anyhow/type

Illustrates the `cloned` method for `Result<&T, E>`, which maps a `Result` containing a reference to a value to a `Result` containing a cloned value. This requires the type `T` to implement the `Clone` trait.

```Rust
let val = 12;
let x: Result<&i32, i32> = Ok(&val);
assert_eq!(x, Ok(&12));
let cloned = x.cloned();
assert_eq!(cloned, Ok(12));
```

--------------------------------

### Add Dynamic Context to Result<T, E> in Rust

Source: https://docs.rs/anyhow/latest/anyhow/trait

Demonstrates using `with_context` to add dynamically generated context to a `Result<T, E>`. The closure provides the context message, allowing for runtime-specific error details.

```Rust
use anyhow::{Context, Result};
use std::fmt::Display;
use std::error::StdError;

// Assuming a function that returns a Result with a compatible error type
fn operation_that_might_fail<T, E: StdError + Send + Sync + 'static>() -> Result<T, E> {
    // Placeholder for an operation that could fail
    unimplemented!()
}

fn demo_with_context_result<T, E: StdError + Send + Sync + 'static>() -> Result<T, anyhow::Error> {
    let error_code = 500;
    let result: Result<T, E> = operation_that_might_fail();
    result.with_context(|| format!("Operation failed with status code {}", error_code))
}
```

--------------------------------

### Rust: Downcast Errors with anyhow

Source: https://docs.rs/anyhow/latest/anyhow/index

Illustrates how to downcast an `anyhow::Error` to a specific error type using `downcast_ref()`. This allows for pattern matching and handling of specific error variants, such as `DataStoreError`.

```Rust
// If the error was caused by redaction, then return a
// tombstone instead of the content.
match root_cause.downcast_ref::<DataStoreError>() {
    Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)),
    None => Err(error),
}
```

--------------------------------

### Rust Result unwrap_err Method

Source: https://docs.rs/anyhow/latest/anyhow/type

Demonstrates the `unwrap_err` method to retrieve the Err value from a Result. It panics if the Result is Ok. This is useful when you expect an error.

```Rust
let x: Result<u32, &str> = Ok(2);
x.unwrap_err(); // panics with `2`
```

```Rust
let x: Result<u32, &str> = Err("emergency failure");
assert_eq!(x.unwrap_err(), "emergency failure");
```

--------------------------------

### Rust: Convert to String

Source: https://docs.rs/anyhow/latest/anyhow/struct

Converts the value into a `String`. This implementation requires the type to implement the `Display` trait, enabling string representation.

```rust
fn to_string(&self) -> String
where Self: Display + ?Sized
```

--------------------------------

### Rust: Provide Default on Error with `or`

Source: https://docs.rs/anyhow/latest/anyhow/type

The `or` function returns the `Ok` value of the first `Result` if it's `Ok`, otherwise returns the second `Result`. Arguments are eagerly evaluated.

```Rust
let x: Result<u32, &str> = Ok(2);
let y: Result<u32, &str> = Err("late error");
assert_eq!(x.or(y), Ok(2));

let x: Result<u32, &str> = Err("early error");
let y: Result<u32, &str> = Ok(2);
assert_eq!(x.or(y), Ok(2));

let x: Result<u32, &str> = Err("not a 2");
let y: Result<u32, &str> = Err("late error");
assert_eq!(x.or(y), Err("late error"));

let x: Result<u32, &str> = Ok(2);
let y: Result<u32, &str> = Ok(100);
assert_eq!(x.or(y), Ok(2));
```

--------------------------------

### Add Dynamic Context to Option<T> in Rust

Source: https://docs.rs/anyhow/latest/anyhow/trait

Illustrates using `with_context` to add context to an `Option<T>` where the context message is generated dynamically by a closure. This is useful when the context depends on runtime values.

```Rust
use anyhow::{Context, Result};
use std::fmt::Display;

fn maybe_get<T>() -> Option<T> {
    // Placeholder for a function that might return None
    None
}

fn demo_with_context_option<T>() -> Result<T, anyhow::Error> {
    let value = 10;
    let t = maybe_get::<T>().with_context(|| format!("Value was {}, but no T found", value))?;
    Ok(t)
}
```

--------------------------------

### Map Result to Default Value (Rust)

Source: https://docs.rs/anyhow/latest/anyhow/type

The `map_or_default` method maps a Result to a U type by applying a function to the Ok value, or returns the default value of U if the Result is Err. This is a nightly-only experimental API.

```Rust
#![feature(result_option_map_or_default)]

let x: Result<_, &str> = Ok("foo");
let y: Result<&str, _> = Err("bar");

assert_eq!(x.map_or_default(|x| x.len()), 3);
assert_eq!(y.map_or_default(|y| y.len()), 0);
```

--------------------------------

### Access Backtrace (Rust)

Source: https://docs.rs/anyhow/latest/anyhow/struct

Retrieves the backtrace associated with an anyhow error. Backtraces are captured when specific environment variables (`RUST_LIB_BACKTRACE=1` or `RUST_BACKTRACE=1`) are set.

```Rust
anyhow = { version = "1.0", features = ["backtrace"] }
```

--------------------------------

### Iterate Over Ok Value (Rust)

Source: https://docs.rs/anyhow/latest/anyhow/type

The `iter` method returns an iterator that yields a reference to the contained value if the Result is Ok, otherwise it yields nothing. This is useful for treating Result like an optional collection.

```Rust
let x: Result<u32, &str> = Ok(7);
assert_eq!(x.iter().next(), Some(&7));

let x: Result<u32, &str> = Err("nothing!");
assert_eq!(x.iter().next(), None);
```

--------------------------------

### Create Error from Message (Rust)

Source: https://docs.rs/anyhow/latest/anyhow/struct

Creates a new error object from a printable error message. This is equivalent to using the `anyhow!` macro but can be useful in function contexts like iterator combinators.

```Rust
use anyhow::{Error, Result};
use futures::stream::{Stream, StreamExt, TryStreamExt};

async fn demo<S>(stream: S) -> Result<Vec<Output>>
where
    S: Stream<Item = Input>,
{
    stream
        .then(ffi::do_some_work) // returns Result<Output, &str>
        .map_err(Error::msg) // Use Error::msg to convert &str to anyhow::Error
        .try_collect()
        .await
}
```

--------------------------------

### Rust Macro ensure Definition

Source: https://docs.rs/anyhow/latest/anyhow/macro

Defines the 'ensure' macro in Rust, which allows for early return of an error if a condition is false. It supports literal messages, formatted messages, and custom error types.

```Rust
macro_rules! ensure {
    ($cond:expr $(,)?) => { ... };
    ($cond:expr, $msg:literal $(,)?) => { ... };
    ($cond:expr, $err:expr $(,)?) => { ... };
    ($cond:expr, $fmt:expr, $($arg:tt)*) => { ... };
}
```

--------------------------------

### Rust Result is_ok

Source: https://docs.rs/anyhow/latest/anyhow/type

Checks if the Result is Ok. Returns true if the result is Ok, false otherwise.

```Rust
let x: Result<i32, &str> = Ok(-3);
assert_eq!(x.is_ok(), true);

let x: Result<i32, &str> = Err("Some error message");
assert_eq!(x.is_ok(), false);
```

--------------------------------

### Rust: Convert Result to Error without Panic

Source: https://docs.rs/anyhow/latest/anyhow/type

The `into_err` function returns the contained `Err` value, never panicking. It's a nightly-only experimental API useful as a maintainability safeguard.

```Rust
fn only_bad_news() -> Result<!, String> {
    Err("Oops, it failed".into())
}

let error: String = only_bad_news().into_err();
println!("{error}");
```

--------------------------------

### Rust Result as_mut

Source: https://docs.rs/anyhow/latest/anyhow/type

Converts a mutable reference to a Result<&mut T, &mut E> into a Result<&mut T, &mut E>. Allows modification of the contained value or error.

```Rust
fn mutate(r: &mut Result<i32, i32>) {
    match r.as_mut() {
        Ok(v) => *v = 42,
        Err(e) => *e = 0,
    }
}

let mut x: Result<i32, i32> = Ok(2);
mutate(&mut x);
assert_eq!(x.unwrap(), 42);

let mut x: Result<i32, i32> = Err(13);
mutate(&mut x);
assert_eq!(x.unwrap_err(), 0);
```

--------------------------------

### Rust Result err

Source: https://docs.rs/anyhow/latest/anyhow/type

Converts a Result<T, E> into an Option<E>, consuming the Result and discarding the success value if present.

```Rust
let x: Result<u32, &str> = Ok(2);
assert_eq!(x.err(), None);

let x: Result<u32, &str> = Err("Nothing here");
assert_eq!(x.err(), Some("Nothing here"));
```

--------------------------------

### Result::copied Method for &mut T

Source: https://docs.rs/anyhow/latest/anyhow/type

Shows the `copied` method for `Result<&mut T, E>`, which maps a `Result` containing a mutable reference to a value to a `Result` containing the copied value. This requires the type `T` to implement the `Copy` trait.

```Rust
let mut val = 12;
let x: Result<&mut i32, i32> = Ok(&mut val);
assert_eq!(x, Ok(&mut 12));
let copied = x.copied();
assert_eq!(copied, Ok(12));
```

--------------------------------

### Rust Result is_ok_and

Source: https://docs.rs/anyhow/latest/anyhow/type

Checks if the Result is Ok and if the contained value satisfies a given predicate. Returns true if both conditions are met, false otherwise.

```Rust
let x: Result<u32, &str> = Ok(2);
assert_eq!(x.is_ok_and(|x| x > 1), true);

let x: Result<u32, &str> = Ok(0);
assert_eq!(x.is_ok_and(|x| x > 1), false);

let x: Result<u32, &str> = Err("hey");
assert_eq!(x.is_ok_and(|x| x > 1), false);

let x: Result<String, &str> = Ok("ownership".to_string());
assert_eq!(x.as_ref().is_ok_and(|x| x.len() > 1), true);
println!("still alive {:?}", x);
```

--------------------------------

### Inspect Ok Value (Rust)

Source: https://docs.rs/anyhow/latest/anyhow/type

The `inspect` method calls a provided closure with a reference to the contained value if the Result is Ok, returning the original Result. It's used for side effects.

```Rust
let x: u8 = "4"
    .parse::<u8>()
    .inspect(|x| println!("original: {x}"))
    .map(|x| x.pow(3))
    .expect("failed to parse number");
```

--------------------------------

### Rust Result map

Source: https://docs.rs/anyhow/latest/anyhow/type

Maps a Result<T, E> to Result<U, E> by applying a function to the contained Ok value, leaving Err values untouched. Useful for composing operations on Results.

```Rust
let line = "1\n2\n3\n4\n";

for num in line.lines() {
    match num.parse::<i32>().map(|i| i * 2) {
        Ok(n) => println!("{n}"),
        Err(..) => {} 
    }
}
```

--------------------------------

### Rust: Clone Result

Source: https://docs.rs/anyhow/latest/anyhow/type

Implements the Clone trait for Result, allowing for duplication of the value if both the Ok and Err types are themselves Clone.

```Rust
impl<T, E> Clone for Result<T, E>
where T: Clone,
      E: Clone,
{
    fn clone(&self) -> Result<T, E>;
    fn clone_from(&mut self, source: &Result<T, E>);
}
```

--------------------------------

### Convert Result<T, E> to Result<&T::Target, &E> (Rust)

Source: https://docs.rs/anyhow/latest/anyhow/type

The `as_deref` method converts a Result containing a type that implements Deref into a Result containing a reference to the dereferenced target type. It handles both Ok and Err variants.

```Rust
let x: Result<String, u32> = Ok("hello".to_string());
let y: Result<&str, &u32> = Ok("hello");
assert_eq!(x.as_deref(), y);

let x: Result<String, u32> = Err(42);
let y: Result<&str, &u32> = Err(&42);
assert_eq!(x.as_deref(), y);
```

--------------------------------

### Rust: Iterate over Result value

Source: https://docs.rs/anyhow/latest/anyhow/type

Implements IntoIterator for Result, enabling iteration over the contained value. The iterator yields the Ok value once if present, otherwise it yields nothing.

```Rust
impl<T, E> IntoIterator for Result<T, E>
{
    type Item = T;
    fn into_iter(self) -> IntoIter<T>;
}

// Example 1: Iterating over Ok
let x: Result<u32, &str> = Ok(5);
let v: Vec<u32> = x.into_iter().collect();
assert_eq!(v, [5]);

// Example 2: Iterating over Err
let x: Result<u32, &str> = Err("nothing!");
let v: Vec<u32> = x.into_iter().collect();
assert_eq!(v, []);
```

--------------------------------

### Map Result with Fallback Function (Rust)

Source: https://docs.rs/anyhow/latest/anyhow/type

The `map_or_else` method maps a Result to a new type by applying a fallback function to the Err value or a mapping function to the Ok value. This allows for lazy evaluation of arguments.

```Rust
let k = 21;

let x : Result<_, &str> = Ok("foo");
assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);

let x : Result<&str, _> = Err("bar");
assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
```

--------------------------------

### Rust Result expect_err Method

Source: https://docs.rs/anyhow/latest/anyhow/type

Shows the `expect_err` method, which returns the Err value from a Result, panicking with a custom message if it's an Ok. This is useful for asserting that an operation failed.

```Rust
let x: Result<u32, &str> = Ok(10);
x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
```

--------------------------------

### Rust: Into Target Type

Source: https://docs.rs/anyhow/latest/anyhow/struct

Converts the type `T` into another type `U` using the `From<T>` implementation for `U`. This is a common pattern for type transformations.

```rust
fn into(self) -> U
where U: From<T>
```

--------------------------------

### Rust Function: with_context

Source: https://docs.rs/anyhow/latest/anyhow/trait

The `with_context` method from the `Context` trait in the anyhow crate. It wraps an error value with additional context that is evaluated lazily only when an error occurs.

```Rust
fn with_context<C, F>(self, f: F) -> Result<T, Error>
where C: Display + Send + Sync + 'static, F: FnOnce() -> C,
Wrap the error value with additional context that is evaluated lazily only once an error does occur.
```

--------------------------------

### Rust: Hash Result

Source: https://docs.rs/anyhow/latest/anyhow/type

Implements the Hash trait for Result, allowing hashing of the contained value if both the Ok and Err types implement Hash. Also provides a method to hash slices of Results.

```Rust
impl<T, E>
where T: Hash,
      E: Hash,
{
    fn hash<__H>(&self, state: &mut __H)
    where __H: Hasher;
    fn hash_slice<H>(data: &[Self], state: &mut H)
    where H: Hasher,
          Self: Sized;
}
```

--------------------------------

### Add Context to Error (Rust)

Source: https://docs.rs/anyhow/latest/anyhow/struct

Wraps an existing error value with additional context, typically a formatted string. This is useful for providing more information when an error is propagated.

```Rust
use anyhow::Result;
use std::fs::File;
use std::path::Path;

struct ParseError {
    line: usize,
    column: usize,
}

fn parse_impl(file: File) -> Result<T, ParseError> {
    // ... implementation details ...
    unimplemented!()
}

pub fn parse(path: impl AsRef<Path>) -> Result<T> {
    let file = File::open(&path)?;
    parse_impl(file).map_err(|error| {
        let context = format!(
            "only the first {} lines of {} are valid",
            error.line,
            path.as_ref().display(),
        );
        anyhow::Error::new(error).context(context) // Add context to the error
    })
}
```

--------------------------------

### Rust: Chain Fallible Operations with `and_then`

Source: https://docs.rs/anyhow/latest/anyhow/type

The `and_then` function calls a closure if the `Result` is `Ok`, returning the closure's `Result`. Otherwise, it returns the `Err` value. It's useful for chaining fallible operations.

```Rust
fn sq_then_to_string(x: u32) -> Result<String, &'static str> {
    x.checked_mul(x).map(|sq| sq.to_string()).ok_or("overflowed")
}

assert_eq!(Ok(2).and_then(sq_then_to_string), Ok(4.to_string()));
assert_eq!(Ok(1_000_000).and_then(sq_then_to_string), Err("overflowed"));
assert_eq!(Err("not a number").and_then(sq_then_to_string), Err("not a number"));
```

```Rust
use std::{io::ErrorKind, path::Path};

// Note: on Windows "/" maps to "C:\\"
let root_modified_time = Path::new("/").metadata().and_then(|md| md.modified());
assert!(root_modified_time.is_ok());

let should_fail = Path::new("/bad/path").metadata().and_then(|md| md.modified());
assert!(should_fail.is_err());
assert_eq!(should_fail.unwrap_err().kind(), ErrorKind::NotFound);
```
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