Erratic /ɪˈrætɪk/

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.

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.

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.

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(())
}

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. All compose orthogonally.

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, 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.

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, and is verified by Miri.

(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.