Can you elaborate? You can't stop people simulating exceptions with sum types, and if you have exceptions, why wouldn't you want them to be typed?
What is presumably talked about is that Haskell also has actual exceptions, generated by calling e.g. `error` or `undefined`.
The semantics of these are.. interesting, mostly thanks to lazy evaluation. For example, `fst (5, error "second") ` is safe to evaluate because the second half of the tuple is a thunk and does not get evaluated. Additionally, there is, to my knowledge, no way to handle exceptions in pure code, presumably due to the undefined evaluation order.
That said, I'm not sure what the alternative would be, because a function like !! (list indexing) can fail and dealing with its fallibility would be a big burden on the programmer.
Yes, exactly.
> The semantics of these are.. interesting, mostly thanks to lazy evaluation. For example, `fst (5, error "second") ` is safe to evaluate because the second half of the tuple is a thunk and does not get evaluated
Correct, and the semantics of loops is also.. interesting. For example `fst (5, last [1..])` is also safe to evaluate.
> What is presumably talked about is that Haskell also has actual exceptions, generated by calling e.g. `error` or `undefined`.
Well, I'm not sure, that's why I asked. I'm trying to understand what astrange meant by "it has exceptions which are a bad language feature, and typed throws which are a worse one". (Throwing exceptions from pure code should be left to such cases, that are impossible to recover from, in my opinion.)
> there is, to my knowledge, no way to handle exceptions in pure code, presumably due to the undefined evaluation order
Correct
> That said, I'm not sure what the alternative would be, because a function like !! (list indexing) can fail and dealing with its fallibility would be a big burden on the programmer.
Indeed. Even more so, what is one supposed to do when an invariant has been violated due to a programming error and there's no way to make progress? Haskell's exceptions are essential. It's even better when they're used in a well typed, well scoped manner, such as provided by my effect library Bluefin
https://hackage.haskell.org/package/bluefin-0.0.6.0/docs/Blu...
That's why I wanted to understand more about what astrange meant. It doesn't match my understanding!
I haven't used Bluefin, but don't Bluefin exceptions suffer from the same issue where it, essentially, 'infects' the program flow? For example, `fst (5, error "second")` seems like it would translate to `fst $ (5,) <$> throw e "second"`, where you would also need to pull an `e` from somewhere. More realistic, something like head would probably be quite awkward:
head e [] = throw e "head: empty list" head e (x:xs) = pure x
You now need to pass in the exception handle, and the return value is now `Eff es a` instead of `a`. This means that you cannot just use the result, so you will likely need to fill your program with monadic stuff like do-notation, <$> and <$>, complicating the program flow and likely also reducing laziness.
fst (5, error "second")
head [] = error "empty list"
head (x:xs) = x
* 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).
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.)
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.