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This is elegantly solved with sum types. (And arguably needs sub typing.)

If instead of just returning (int, error) the function would return (int, parseError | outOfBoundsError) you would know that the parser function can fail on reading a number at all and on the number being to big/small to fit the type and handle then accordingly.

Saliently, Java in a sense has supported sum types in the throws declaration and the subsequent catch statements forever. Unfortunately it has not landed in other places where you can use types so you cannot use it for returning errors. Scala 3 supports this but has tiny adoption it seems.

by kitd13 hours ago|

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That's helped by errors.Unwrap() and errors.Is(), though you do need to build and structure your errors appropriately.

by adrianmsmith13 hours ago|

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It's helped, but that's runtime behaviour. The compiler can't help you with questions like "you didn't handle certain errors" or "the error you're trying to check can never occur".

Nor does that help with documentation, I'm guessing if I read a file then it might raise the error that the file does not exist. But what exactly is the technical type of that error? You have to search through the function's implementation, and functions that function calls, etc., to find out.

And it doesn't help with refactoring. If you create new.FileNotFoundError and change your function to return it, existing code which checks for old.FileNotFoundError will start to silently fail.