---
title: anyhow
summary: Construct an ad-hoc error from a string or existing non-anyhow error value.
url: https://www.fastly.com/documentation/reference/compute/sdks/rust/fastly/error/macro.anyhow/
---

**Kind:** Macro  
**Module:** [error](https://www.fastly.com/documentation/reference/compute/sdks/rust/fastly/error/)  
**Package:** fastly 0.13.1

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

Construct an ad-hoc error from a string or existing non-`anyhow` error
value.

This evaluates to an [`Error`][crate::Error]. It can take either just a
string, or a format string with arguments. It also can take any custom type
which implements `Debug` and `Display`.

If called with a single argument whose type implements `std::error::Error`
(in addition to `Debug` and `Display`, which are always required), then that
Error impl's `source` is preserved as the `source` of the resulting
`anyhow::Error`.

## Example

```rust
use anyhow::{anyhow, Result};

fn lookup(key: &str) -> Result<V> {
    if key.len() != 16 {
        return Err(anyhow!("key length must be 16 characters, got {:?}", key));
    }

    // ...
}
```