Hey.
So I have been going over some of the posts over various forums discussing this issue but couldn't come to any reasonable conclusion because there is no perfect answer.
here is the problem:
The Problem: "Phantom" Error Variants
Imagine an HTTP client crate with this error enum:
pub enum ClientError {
UrlError(String),
RequestError(String),
ResponseError(ResponseErrorInfo),
SerializationError(String),
IoError(IoError),
HeaderError(String),
CookieError(String),
UnknownError(String),
}
Now consider two functions:
signup(): Can returnRequestErrorResponseErrorSerializationErrorCookieErrorchallenge(): Returns OnlyResponseError,RequestError.
Now imagine that various other functions can have various combinations of these...
The Issue:
If both return ClientError, users handling challenge() must account for all 8 variants, even though only 2 are possible. Sure, they can use _ => {}, but now:
- Error handling becomes guesswork ("Which errors actually happen here?")
- Documentation lies ("Says it can return
IoError, but can it really?")
Approach 1: One Big Error Enum (The "Easy" Path)
// Pros: Simple propagation with ?, clean imports
impl fetch_user() -> Result<User, ClientError> {
let res = make_request()?; // Auto-converts to RequestError
parse_response(res)?; // Auto-converts to ResponseError
Ok(user)
}
Pros:
- Dead-simple error returns
- No conversion boilerplate
- Unified API surface
Cons:
- Forces users to handle nonexistent errors:
match error {
ClientError::RequestError(e) => log(e),
ClientError::ResponseError(e) => retry(),
_ => {} // Wait... can this even happen? 🤔
}
- Becomes a maintenance nightmare as the crate grows
Approach 2: Granular Errors (Composition Chaos)
Split into smaller enums:
// Base errors
#[derive(Debug)]
pub struct RequestError(String);
#[derive(Debug)]
pub struct ResponseError(String);
#[derive(Debug)]
pub struct SerializationError(String);
// First layer combo
pub enum RequestResponseError {
Request(RequestError),
Response(ResponseError),
}
// Second layer combo
pub enum RequestResponseSerializeError {
RequestResponse(RequestResponseError),
Serialize(SerializationError),
}
Error Propagation Without Magic:
// Manual From implementations required
impl From<RequestError> for RequestResponseError {
fn from(e: RequestError) -> Self {
Self::Request(e)
}
}
impl From<ResponseError> for RequestResponseError {
fn from(e: ResponseError) -> Self {
Self::Response(e)
}
}
// Now a function that uses these
fn fetch_data() -> Result<String, RequestResponseError> {
let data = make_request()?; // ? works because we implemented From
let parsed = parse_response(data)?;
Ok(parsed)
}
Matching Becomes Russian Dolls:
match error {
RequestResponseSerializeError::RequestResponse(
RequestResponseError::Request(RequestError(msg))
) => println!("Request failed: {}", msg),
RequestResponseSerializeError::RequestResponse(
RequestResponseError::Response(ResponseError(msg))
) => println!("Response failed: {}", msg),
RequestResponseSerializeError::Serialize(
SerializationError(msg)
) => println!("Serialize failed: {}", msg),
}
Pros:
- Type safety: Functions clearly declare their possible failures
- No phantom variants haunting your matches
Cons:
// This is what hell looks like in Rust:
1. 3 error types Ă— 2 conversions each = 6 manual From impls
2. Display impls? That's another 3 Ă— display implementations!
3. Every new error combination requires a new enum + conversions
4. Matching becomes digsite archaeology: "Error::Layer1(Layer2::Variant(...))"
Approach 3: Overlapping Enums (The Fragile Middle Ground)
Create overlapping enums for different functions:
// For update_user()
pub enum UpdateError {
Request(String),
Response(ResponseErrorInfo),
Serialization(String),
}
// For delete_user()
pub enum DeleteError {
Request(String),
Response(ResponseErrorInfo),
Io(IoError),
}
Pros:
- Precise per-function errors
- No phantom variants
Cons:
- DRY Violation Central: If
Requestchanges, you must update every enum containing it - Conversion Madness: Need manual
Fromimpls between similar enums - API becomes a maze of error types
What Would You Do?
I'm torn between:
- The "Easy" Lie (One enum, but pretend some variants don't exist)
- Boilerplate Purgatory (Precision at the cost of code bloat)
- Fragile Overlaps (Maintenance issues)
Is there a better way?
What is the idiomatic way to handle this?
Out of all of these the 3rd seems to be the most straight forward and least painful, but I would love to have some good alternatives that don't sacrifice readability and doesn't go into some insane language patterns that few people know.
(Code examples simplified for clarity)