Sure, exceptions are very different to Result-style error handling - even checked exceptions. Here are some differences:
* Errors must be explicitly listed as part of the function signature. Checked exceptions and the equivalent for exceptions but they are rarely used in practice. I think Android uses them, but they were so unpopular in C++ that they removed them from the language!
* The syntax to catch and handle errors is very different and more more verbose for exceptions. It can also make flow control a real pain in some languages where you can't declare a variable outside the try body (e.g. references in C++).
* Result errors need to be explicitly handled whereas exceptions are silently propagated by default.
Even though they're similar enough that you could translate one to the other in most cases, they're different enough that saying one is "an emulation" of the other is just stupid.
In my experience Result-based handling is far superior with two exceptions:
1. In functional code like map & filter where it can become quite awkward to explicitly deal with returning errors.
2. It's hard to get a stack trace from where the Err was created rather than from where it was unwrapped. Less of a problem with exceptions which record a stack trace from where they were thrown (in most languages anyway - C++ is an annoying exception).
> saying one is "an emulation" of the other is just stupid.
I did say that indeed. Am I to conclude doing so was stupid? If so I would find that very rude.
(For what it's worth I was trying to understand what astrange meant by "it has exceptions which are a bad language feature, and typed throws which are a worse one" and offering that characterization as a way of trying to tease out exactly what he/she meant. Your response contains many interesting points and I would otherwise be interested in discussing with you further, but I'm not too inclined to now that you have suggested you might think I'm stupid.)
They don't get stack traces, for one. (That's arguably the biggest problem with Rust: .unwrap() gives you a stack trace, but has problems; whereas ? erases your stack trace.)
In principle, static analysis could identify unhandled exceptions, then trace the exception, then make that information available to the top-level "Err returned from main" handler. In practice, that's never going to happen in Rust.
Sum types can have stack traces by adding a stack trace on creation. `Result<(), &'static str>` does not have a stack trace, but `Result<(), std::backtrace::Backtrace>` sure does. YMMV as to whether it makes sense to do that in any particular circumstance.
Sure, if your definition of exceptions includes "must include a stack trace", then sum types can't simulate exceptions. But by that definition Haskell hasn't had exceptions until the last year or two. You might agree with that (I don't) but I'm trying to understand astrange, who said "[Haskell] has exceptions which are a bad language feature, and typed throws which are a worse one". It seems doubtful that "having a stack trace" is part of what he/she considers bad about exceptions, so that aspect is probably not relevant to my line of questioning. What exactly is bad about exceptions? That's the point of me forking off this thread. So far no one has offered an answer.
* Errors must be explicitly listed as part of the function signature. Checked exceptions and the equivalent for exceptions but they are rarely used in practice. I think Android uses them, but they were so unpopular in C++ that they removed them from the language!
* The syntax to catch and handle errors is very different and more more verbose for exceptions. It can also make flow control a real pain in some languages where you can't declare a variable outside the try body (e.g. references in C++).
* Result errors need to be explicitly handled whereas exceptions are silently propagated by default.
Even though they're similar enough that you could translate one to the other in most cases, they're different enough that saying one is "an emulation" of the other is just stupid.
In my experience Result-based handling is far superior with two exceptions:
1. In functional code like map & filter where it can become quite awkward to explicitly deal with returning errors.
2. It's hard to get a stack trace from where the Err was created rather than from where it was unwrapped. Less of a problem with exceptions which record a stack trace from where they were thrown (in most languages anyway - C++ is an annoying exception).