Make Your Errors Clearer By Splitting Them In Half

Ryan James Spencer

April 6 2020, 8:26PM

Are your errors type devolving into grab bags with varying degrees of categorization? Frequently who is at fault is not clear, and that can be one of the most ergonomically essential classifications. If a program makes it clear that an error is due to a user mistake, an internal complication, or possibly a bug, that dramatically eases the usability of the service/library/tool in question.

Making this clear is trivial with a top-level enum. I'm going to pretend we an interface with two sides in question; the program authors (the providers) and the users of the program (the consumers). For simplicity, I've called these External and Internal, but it could also be Provider and Consumer or whatever makes the designation clear for your use case.

mod error {
  pub enum ExternalError {
    // e.g. MalformedInput, MissingArguments, and so on.
  }

  pub enum InternalError {
    // e.g. IoError, SerdeError, and so on.
  }

  pub enum Error {
    External(ExternalError),
    Internal(InternalError),
  }
}

A sum type lets you easily pattern match and analyze the error itself without fickle operations or messy validation logic. Sum types (enums) are fantastic, and you should be looking for ways to leverage them whenever possible.

A mental model for this I like is thinking every error has an owner. Then you can write functions that have clear offenders. Then, by signature, you are assured that a module only deals with internally related failures or externally associated concerns.

I love this approach because when you get to debugging, you can quickly ascertain if an error is from mishandling, some operational concern, or perhaps a bug. The user sees each diagnostic without any ambiguity to who is at fault. In the same vein that a bug may be breaking an invariant, we might have an Invariant case, which stipulates that an invariant has been breached, without necessarily having to reach for assertions, or hoping that debug_assertions will fire in tests. And by all means, if there are more than two offenders, it is best to define them clearly!


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