# Erratic /ɪˈrætɪk/
[](https://github.com/lansyin/erratic)
[](https://crates.io/crates/erratic)
[](https://docs.rs/erratic/latest/erratic/)
This library provides `Error<S = Stateless>`, an error type with **optional** dynamic dispatch,
enabling applications to handle errors uniformly across different contexts.
## Quick Start
In most cases, `Error` can serve as a drop-in replacement for `Box<dyn Error>`.
Compared to the latter, it occupies only 1 usize, making the happy path faster.
```rust
use erratic::*;
fn write(filename: &str) -> Result<()> {
File::open(filename)?.write_all(b"Hello, World!")?;
Ok(())
}
```
## Attaching Context & Payload
When constructing an error, you can optionally attach a static context and/or a dynamic payload.
If attached, the memory is merged into a single allocation when the source error is materialized.
If only a context is provided, no heap allocation occurs at all.
```rust
use erratic::*;
fn read_weak(r: &mut Weak<Reader>, buf: &mut [u8]) -> Result<u64> {
if buf.is_empty() {
return mkres!("buf must not be empty"); // No alloc so long as no format args.
}
let r = r.upgrade()
.with_context(literal!("stream expired"))?; // No alloc.
// .with_payload("stream expired")?;
let n = r.read(buf)
.with_context(literal!("failed to read from stream: "))
.with_payload(r.id())?; // Alloc once for error, id, and context.
// .with_payload_fn(|| format!("failed to read from stream: {}", w.id()))?;
Ok(n)
}
```
## Binding State
When propagating an error that requires special handling, you can attach a generic state to it.
If the state is small enough and neither the source error, context, nor payload is attached,
the state is inlined without any heap allocation.
```rust
use erratic::*;
#[derive(Debug)]
enum State { RetryLater }
fn try_write(w: &mut Writer, data: &[u8; 64]) -> Result<(), Error<State>> {
w.reserve_chunk(64)
.ok()
.with_state(State::RetryLater)?; // No alloc.
w.write(data)
.with_context(literal!("failed to write to stream: "))
.with_payload(w.id())?;
Ok(())
}
```
The state is optional. When no runtime state is actually stored, errors can be cheaply converted between
different state types. This means infrastructure errors cross any number of layers with a single allocation,
domain errors avoid the heap entirely, and both share the same `Error<S>` type, so they compose orthogonally.
```rust
fn write(w: &mut Writer, data: &[u8; 64]) -> Result<()> {
while let Err((state, _)) = try_write(w, data).extract_state()? {
match state {
State::RetryLater => {
thread::yield_now();
}
}
}
Ok(())
}
```
The `?` operator covers the most common cases, notably including conversion from `Error` to `Error<S>`:
- `impl Error` -> `Error`
- `impl Error` -> `Error<S>`
- `Error` -> `Error<S>`
Stateful errors are meant to be handled explicitly. Several utility methods are provided:
- `erase_error()?`: Propagate the error.
- `extract_state()?`: Take the state out, or propagate the error.
- `map_state()?`: Transform the state with a closure.
- `lift_state()?`: Transform via `From<S>`.
## Backtrace
When the `backtrace` feature is enabled and backtrace capture is configured via
[environment variables][backtrace-conf], `Error<S>` automatically captures a backtrace if there isn't
already one in the source chain. The backtrace will be appended after the error chain during debug
formatting, unless the minus sign, e.g. `{:-?}`, is specified to suppress it.
[backtrace-conf]: https://doc.rust-lang.org/std/backtrace/index.html#environment-variables
## Representation
If the error contains only a source, the error message is inherited from the source. Otherwise, the
error message is constructed from other attached components.
```
<error> ::= <source>
| <state>": "<context><payload>
| <state>": "<context>
| <state>": "<payload>
| <context><payload>
| <context>
| <payload>
| <state>
```
By default, only the top-level error is shown during formatting. To display the full error chain,
format with alternate or debug specifiers.
- `{}`: Displays only the top-level error.
- `{:#}`: Displays the full error chain.
- `{:?}`: Displays the full error chain with backtrace (if captured).
- `{:#?}`: Displays all information in a struct-like format.
The error chain is defined as follows:
```
<chain> ::= <error>
| <error>"\n -> "<chain>
```
## Layout
Type-wise, `Error<S>` is an internally tagged union, and it requires pointers to be aligned to 4 bytes,
freeing up the lower 2 bits to encode its discriminant. Pointer tagging in this crate fully follows
[strict provenance][strict_provenance], and is verified by Miri.
[strict_provenance]: https://doc.rust-lang.org/std/ptr/index.html#strict-provenance
```plaintext
(32-bit platform, little-endian)
(Context Only)
[......00|........|........|........]
\
`rodata-> [Context]
(Allocation Required)
[......01|........|........|........]
\
`heap-> [VTable|State|Error|Payload|Context]
(Small State Only)
[00000010| ~ State ~ ]
```
## Contributing
Contributions are warmly welcomed! Whether you have a bug report, feature request, or
an improvement in mind, feel free to open an issue or submit a pull request.
All ideas—big or small—help make this library better for everyone.