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this post was submitted on 10 Aug 2024
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The problem is that most languages with exceptions treat that as the idiomatic error mechanism. So checked exceptions were invented, essentially, to do exactly what you say: add the exception type to the function signature.
Having separate errors-as-return-values and unwinding-for-emergencies is a much more recent trend (and, IMO, an obviously good development).
I'm not sure why it's "obviously" good to move from one mechanism to two: as a user I now have to categorise every path to work out which is appropriate.
What I said was less about adding to a function signature than it was about adding to a facade - that is, a system boundary, although the implementation may be the same depending on language. People typically use exceptions pretty badly - a function signature with a baggage-train of internal exceptions that might be thrown by implementation guts is another antipattern that gives the approach a bad rep. Errors have types too (or they should have), and the typical exception constructor has a wrapper capability for good reason.
I should have stressed the "opinion" part of "IMO". What I mean is that, when I read this, it struck me immediately as being exactly correct: https://joeduffyblog.com/2016/02/07/the-error-model/
That's a cracking article.
My own use of jvm errors tends to follow the same kinds of patterns: I think the major fault with that model is having RuntimeException as a subclass of Exception, because it's really intended for abandonment-style errors. (The problem is that lots of people use it instead as an exception system in order to cut down on boilerplate.)
I find it eye-opening that the author prefers callsite annotation with
try(although I'm not going to argue with their experience at the time). I can see this being either "no big deal" or even "a good thing" to Rust users in particular - mutability and borrowing annotations at both callsite and definition aren't required to make the language work afaict (your ide will instantly carp if you miss 'em out) but the increased programmer visibility is typically seen as a good thing. (Perhaps this is down to people largely reviewing PRs in a browser, I dunno.) Certainly there's tons of good food for thought there.I'm a Rust fan, and I do think they eventually struck a pretty good "visibility vs noise" balance with
?(which was highly controversial at the time).I'm coming around to it.