Seeking Advice on Best Practices for Error Handling in OCaml

Hi everyone, :smiling_face_with_three_hearts:

I’m relatively new to OCaml and have been enjoying learning the language over the past few months. One area where I’m still struggling a bit is with error handling. I’ve read about various techniques like using exceptions, the result type, and the option type, but I’m not quite sure when to use each approach.

Here’s a brief overview of my current project:

I’m building a command-line tool that processes data files and outputs reports.
The tool needs to handle various errors, such as file not found, invalid data formats, and user input errors.
https://discuss.ocaml.org/t/are-there-any-ocamls-best-practices
I’m looking for advice on the following:

What are the best practices for handling different types of errors in OCaml?
Are there specific libraries or patterns that can help make error handling more robust and readable?
How do you usually test error scenarios in your OCaml projects?

Thanks

2 Likes

You may find the following two blog posts by @keleshev useful.

https://keleshev.com/composable-error-handling-in-ocaml
https://keleshev.com/advanced-error-handling-in-ocaml

Cheers,
Nicolas

2 Likes

For simple cases, exceptions should be fine for error handling. It sounds like your tool will mostly be dealing with I/O errors anyway, and in OCaml, these always start out as raised exceptions. Since you mentioned you just need to report the errors to the user, you could probably even just run your app under a top-level exception handler that just catches and reports the exceptions you care about.

One protip: make sure you have the environment variable OCAMLRUNPARAM=b. This enables exception stack traces. Otherwise stack traces are turned off.

1 Like

In the MirageOS context, we discussed error handling in 2017, which may be interesting to read https://mirage.io/docs/mirage-3.0-errors (basically: no exceptions (apart from Out of memory and Stack overflow), everything being result). It is a bit tedious since the standard library uses exceptions quite a lot.

1 Like

We are mostly creating non-interactive data processing programs with a deep library structure. Most errors have to propagate to the main program, where the decision is made how to handle them (log to file, log to database, send email, exit using the correct status code, …).

We use the result monad almost exclusively. Combined with the “new” binding operators let* etc. this is very convenient.

In my opinion, exceptions should be reserved for unforeseen situations. I wrote a lot of Python code in the past. They use exceptions for flow control - I always considered this as very wrong.

One kinda hack I do for small projects where I’m feeling lazy is to just emit exceptions and not even catch them but just change the printing formatter:

let () = Printexc.register_printer (function
  | Failure msg -> Some msg
  | _ -> None
)

This was from a static website generator so I’d raise Failure with custom error messages when things went wrong and it formed a passable interface for users.