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Choosing an Error Handling Strategy

From Real World OCaml. The whole page on error handling is good, but I’m mostly interested in Exceptions vs explicit Result/Error types:

Given that OCaml supports both exceptions and error-aware return types, how do you choose between them? The key is to think about the trade-off between concision and explicitness.

Exceptions are more concise because they allow you to defer the job of error handling to some larger scope, and because they don’t clutter up your types. But this concision comes at a cost: exceptions are all too easy to ignore. Error-aware return types, on the other hand, are fully manifest in your type definitions, making the errors that your code might generate explicit and impossible to ignore.

The right trade-off depends on your application. If you’re writing a rough-and-ready program where getting it done quickly is key and failure is not that expensive, then using exceptions extensively may be the way to go. If, on the other hand, you’re writing production software whose failure is costly, then you should probably lean in the direction of using error-aware return types.

To be clear, it doesn’t make sense to avoid exceptions entirely. The maxim of “use exceptions for exceptional conditions” applies. If an error occurs sufficiently rarely, then throwing an exception is often the right behavior.

Also, for errors that are omnipresent, error-aware return types may be overkill. A good example is out-of-memory errors, which can occur anywhere, and so you’d need to use error-aware return types everywhere to capture those. Having every operation marked as one that might fail is no more explicit than having none of them marked.

In short, for errors that are a foreseeable and ordinary part of the execution of your production code and that are not omnipresent, error-aware return types are typically the right solution.

programming ocaml advice

11 months ago

Real World Ocaml 2nd Edition

A preview of the second edition of Real World OCaml is available online.

books ocaml programming ml

4 years ago