Rust Programming @lemmy.ml gomp @lemmy.ml 8mo ago

[newbie] Can I specify what subset of errors a function can return?

I have two functions that are similar but can fail with different errors:

#[derive(Debug, thiserror::Error)]
enum MyError {
  #[error("error a")]
  MyErrorA,
  #[error("error b")]
  MyErrorB,
  #[error("bad value ({0})")]
  MyErrorCommon(String),
}

fn functionA() -> Result<String, MyError> {
  // can fail with MyErrorA MyErrorCommon
  todo!()
}

fn functionB() -> Result<String, MyError> {
  // can fail with MyErrorB MyErrorCommon
  todo!()
}

Is there an elegant (*) way I can express this?

If I split the error type into two separate types, is there a way to reuse the definition of MyErrorCommon?

(*) by "elegant" I mean something that improves the code - I'm sure one could define a few macros and solve that way, but I don't want to go there

edit: grammar (rust grammar)

4 comments

You can't create a subset of an enum directly, but splitting this up into multiple types works. You can have FunctionAError with errors that function can produce and a variant for your common errors, and FunctionBError which is similar:

#[derive(Debug, Error)] enum MyErrorCommon { #[error("bad value ({0})")] MyErrorCommon(String), } #[derive(Debug, Error)] enum FunctionAError { #[error("error a")] MyErrorA, Common(#[from] MyErrorCommon), } // and same for FunctionBError

The try operator (?) will automatically use From impls to convert errors for you as well. If a function returns a result containing MyErrorCommon in your function and you use ? on it, it gets converted to that function's error type for you. thiserror generates the From impl for you if you use #[from].
use-case?
- Different functions whose possible failure reasons have a non-empty intersection, but don't coincide completely (IDK if this clarifies? I think the example code in the OP is clearer)
  
  Yeah, I got that.
  
  I'm asking what would be the benefit of not using a single error enum for all failure reasons?