Why checked exceptions failed

https://borretti.me/article/why-checked-exceptions-failed

by rsaarelm • 3 years ago

108 314 3 years ago

necovek9• 3 years ago

I am surprised an even more obvious interface definition of

  public interface ExampleInterface {
      void foo() throws IOException, AnotherException;
  }

was not covered: is this also not supported?

That would solve a lot of use cases if inheritance is respected.

ailef• 3 years ago

This is indeed the supported way to do it.

necovek9• 3 years ago

Thanks for confirming as I am not familiar with Java, but good to know it does make sense. As such, I am not sure what did the original author complain about because that's perfectly sufficient language support.

masklinn5• 3 years ago

It is supported, but does not actually solve anything, you're still limited to whatever checked exceptions the creator of the interface has decided might be valid (it's just that in the original it's "none").

Let's say that the creator of the interface allows IOException, but the backend of my implementation is a database so I have SQLExceptions, same issue.

Is the creator of the interface supposed to add every checked exception in the standard library? Ignoring that this still isn't all of them, then the caller is hosed because they have to handle (or rethrow) every single one of them which is no better. So they probably go "fuck it" and just handle / rethrow Exception. At which point you can just do that on the interface, and it's not helping anyone, and you're better off just not saying anything.

Checked exceptions simply don't mesh with the language: because the language provides limited to no way to abstract over them they're an issue every time you're trying to be generic over anything, the only situation in which they kinda sorta work is if the entire callchain is concrete, which not only is very limiting but it's very much not idiomatic.

throwaway290• 3 years ago

Maybe your SQLException should be caller's IOException? I don't code Java but IMO callers should be hit with errors at the right abstraction level. If caller is using that interface and not your low-level backend functions directly then caller should expect IOException not SQLException.

If the interface does not provide exception types for all cases so you are stuck rethrowing your error as a wrong one, and I'd say that means the interface is bad but not checked errors are bad...

maratc2• 3 years ago

Let's say that the creator of the interface allows IOException, but the backend of my implementation is a database so I have SQLExceptions, same issue.

The creator of the interface should decide on a generic exception type:

  public interface UniversalStorageInterface {
      void store() throws StorageException;
  }

Then, in the present, the FileStorage can define (and throw) a FileStorageException (inherits from StorageException), and the SqlStorage may define (and throw) a DatabaseStorageException (same).

In the future, where we might want to store everything on a (non-existing yet) Cerulean backend, we would then define (and throw) a CeruleanStorageException (again, an implementation of StorageException), and our basic Interface would not need to change. We would also have no need to recompile FileStorage or SqlStorage (or proprietary SteelBlue storage where we have no code).

PretzelPirate• 3 years ago

Wouldn't it still be a problem? By putting "throws [Exception List]" in the interface, the interface making assumptions about the implementation details of every potential implementation.

The interface can't know every exception that an implementation might throw, and using your example, some implementations may not use IP at all and won't throw those exceptions.

It seems like you'd end up needing to list every possible exception on "foo() " when defining the interface, and then handle every possible exception in any code that uses "ExampleInterface".

At that point, it would be better to not annotate exception information at all.

thewix• 3 years ago

Exceptions should absolutely be a part of an interface and it stops you from leaking implementation details. If I have an IRepository interface then none of the methods should ever throw SqlException, HttpException, or anything like that. You'd map them to something else. Exceptions in most languages are implicitly part of the contract, and not having consistent ones on an interface breaks Liskov's Substitution Principle.

grumbel5• 3 years ago

the interface making assumptions about the implementation details of every potential implementation.

No, as you'd force the implementation to catch their implementation specific exceptions and rethrow them in the way defined by the interface.

That's really the crux of it, when I call foo(), I expect to see a FooException when something goes wrong, since that's the only one I can meaningful react to. If I get an ImplementationDetailException that I never heard from and that isn't specified in the interface, how am I supposed to react to that in a meaningful way?

If exception are supposed to be used for error handling, you have to actually report and handle them in a well specified manner, you can't just treat them as a slightly less crashy version of a SIGSEGV.

necovek9• 3 years ago

As others have pointed out, my example would have been better if it said `ExampleExceptionFoo, ExampleExceptionBar` to more clearly indicate that an interface would define what exceptions it could throw as part of the contract. Individual implementations could extend those base exceptions if they need to.

There is not much difference between this and using error types, except the syntax sugar.

DamonHD• 3 years ago

"Nobody uses [checked exceptions" in Java, this casually claims.

Oh dear. I have a slew of current counter examples on my machine, in GitHub, etc.

Either carelessly untrue or else annoying hypebole.

Made the rest very hard for me to read.

o1y32• 3 years ago

This is a very old topic. Insights from Anders Hejlsberg:

https://www.artima.com/articles/the-trouble-with-checked-exc...

hodgesrm• 3 years ago

Great article with a lot of pithy insights. Here's one of the core issues with checked exceptions. 2nd sentence to the last is the point.

Anders Hejlsberg: Yeah, well, Einstein said that, "Do the simplest thing possible, but no simpler." The concern I have about checked exceptions is the handcuffs they put on programmers. You see programmers picking up new APIs that have all these throws clauses, and then you see how convoluted their code gets, and you realize the checked exceptions aren't helping them any. It is sort of these dictatorial API designers telling you how to do your exception handling. They should not be doing that.

redeeman• 3 years ago

Can I specify that implementations of foo can throw no, some, or all exceptions?

just because theres the possibility of a throw in the signature, does not mean it has to be done in a concrete implementation, and if implementations are really to be exchangable, it has to be handled in case implementation changes.

What would it even mean to write something like throws *?

Roughly similar to "throws Exception" or "throws Throwable" - that it may throw whatever you have under the sun?

Concretely, checked exceptions in Java failed because Java lacks “throwingness polymorphism”, if you will.

it does allow for extending exceptions, so an interface can define some exceptions that are basic, and implementations can then choose to extend those to provide MORE detail if needed.

Was Java wrong to add checked exceptions? No. They took a risk and it didn’t pay off.

Uhm.... Im gonna go ahead and use some other opinion on this

vmaurin• 3 years ago

For checked exception, as you mention, "throws *" is throws Exception, then people can refine it on implementation.

As many language feature, checked exception are often misused and/or misunderstood. Too many time I have seen people blindly propagating checked exception above, and you end up having a controller method exposing a SQLException. Then for sure, it is better to use unchecked exceptions overall

rcme• 3 years ago

The issue with all exceptions, and error handling in general, is that only very rarely is there actually recourse for an error. For instance, in an HTTP request handler, the vast majority of errors will end up as something like a 500. Checked exceptions are annoying because they make you explicitly handle an error when you likely already have a handler in place to handle all exceptions, checked or otherwise.

inglor• 3 years ago

Exactly, the boilerplate overhead isn't worth it when 95% of times you don't have anything meaningful to do with the exception but let it propagate.

computerdork• 3 years ago

To me, you kind of hit both the pros and cons of checked exceptions. It makes the calling code have to deal with an important error case. This is actually super useful for very important error situations (for instance, let's say you're writing some type of file-processing class, and the class was unable to open the file for some reason, it might be a good situation to throw a checked exception as this can probably happen reasonably often).

To me, the difficulty with exceptions is when writing a method that needs to throw an exception is deciding whether it should throw one that is actually checked. In fact, read a few of years ago most exceptions should actually be unchecked, but at least then, this was still greatly up for debate.

... as I type this, am realizing having the option of unchecked and checked exceptions is nice in that they reduce the amount error code you have to write. Because by throwing unchecked exceptions, this is error handling that all the entire calling code-chain doesn't need to deal with. The obvious situations are system errors like out of memory errors, missing resources... but also those programmatic errors that are unexpected, like array out of bounds, ClassCastException.

Also, the calling chain then has the option to handle it if it's needed. For instance, if you're writing a low-level messaging queue that needs to report system errors (btw, this was a real situation that happened to me). Just my two cents.

javajosh3• 3 years ago

Checked exceptions are used to make up for Java's inability to return more than one value, plus it's inability to wrap two values without defining a new type. In other words, I think checked exceptions are basically a symptom of a lack of object literal syntax. This is in addition to their status as a "cool language feature" that is a siren song to new, bright programmers looking to spice up their designs. Exceptions are in general problematic (they move the program counter in a disjoint way - "non-local change of control" I think it's called) so it makes this particular siren song doubly deadly.

It would probably be handy if someone wrote a pamphlet on "Refactoring Exceptions" to give teams the confidence to refactor exceptions out of their code. I'll offer $20 to Martin Fowler to write such a thing.

dudul• 3 years ago

Just came to say this. I think the best way to represent failure is in the return type, but Java made that very cumbersome and noisy.

kaba05• 3 years ago

Exceptions “don’t move the program counter in disjoint ways”, they are part of the “structured gotos”. In fact, it has the same control flow as an early return does, with the handler being locally found in a parent’s (recursively) method body.

Also, the point about multiple return types is pointless — it already has Optional, a proper Return type is completely feasible to implement and use in Java. So is a Pair<A,B> if tuples are what you mean. Some are just not part of the standard lib by default.

javajosh3• 3 years ago

with the handler being locally found in a parent’s (recursively) method body

Yeah, that doesn't sound disjoint /s

Supermancho• 3 years ago

Return type is completely feasible to implement

Yet it is rarely done. Lots of boilerplate to replace what is easily done in other languages. If java provided a native tuple type, new patterns would appeat. As it is, too many lines for 1 off returns. Send off a serialzed json and deserialize it later. Easier than specific dtos.

throw3823423• 3 years ago

But the issue about the multiple return types is not pointless at all: Optional types, and pair types, were added to the language way later than checked exceptions. We didn't even have actual generics back then! So the checked exceptions really were a way to get around lacking alternative error management features.

If backwards compatibility wasn't a concern, checked exceptions would probably go away in a version or two, and we'd have some kind of monadic error type instead. But Java takes this seriously, and the standard library itself has methods with checked exceptions, so the timeline to go from checked exceptions to something else is very long.

spreiti• 3 years ago

FYI: With Java 21 (2023/09/19) this is finally possible.

Relevant JEPs:

- 441: Pattern Matching for switch

- 440: Record Patterns

- 409: Sealed Classes

- 395: Records

It is kind of similar to Scalas version of pattern matching with case classes.

isbvhodnvemrwvn• 3 years ago

And it won't get adopted any time soon due to backwards compatibility and existing code. Anybody who wanted to use it used vavr at this point.

invalidname25• 3 years ago

My opinion is the exact opposite see https://debugagent.com/everything-bad-in-java-is-good-for-yo...

Checked exceptions are unpopular since no one likes responsibility. But they are great when used right. Calls that must have proper cleanup after them e.g. SQL, IO are checked. The fact that this must be communicated via interfaces is hugely important.

There are "weird" problems such as stream close() throwing a checked exception. That's an API misbehavior that has a workaround thanks to try-with-resources.

scarface_7413• 3 years ago

Calls that must have proper cleanup after them e.g. SQL, IO are checked.

Why? In C# you use a “using” block and whether your code exits the using block successfully or with an exception, the cleanup is automatic.

RAII has been around at least since the 80s with C++.

invalidname25• 3 years ago

We have it in Java too. For us it's try-with-resources and has nothing to do with checked exceptions. Checked exceptions remind you that you need to wrap the code and need to take that into account.

They also force you to declare that an exception is thrown if you don't want to handle it in the current method. That's important as the signature of the method carries an important failure that can't be dismissed and is enforced by the compiler.

equalsione• 3 years ago

This is what try-with-resource does in Java. Introduced in Java 8 I think so better late than never

specialist8• 3 years ago

From your OC:

Null is fast. Super fast. Literally free.

Yes and: The JIT will optimize away Null Object method invocations.

All my composable classes have a Null Object. No null checks means concise iteration (eg graph traversal). Eliminates NPEs. Just as fast.

Win / win / win.

Optionals (classes and operators) continue to be turrible mistakes. So much unnecessary effort, so little benefit.

Ditto @Nullable and @Nonnull.

Someday, maybe, I'll make a javac compiler plugin which autogenerates Null Object implementations and converts "Node abc = null" to "Node abc = Node.DEFAULT_NULL_OBJECT_INSTANCE".

https://en.wikipedia.org/wiki/Null_object_pattern

anonymoushn8• 3 years ago

It seems like when one is implementing Null Objects and making incomplete attempts to prevent values from ever being null it is not really an argument in favor of nullable values being good and the only type of values that should be present in a language.

anonymoushn8• 3 years ago

It's weird to argue that null is how the hardware works. Many structs contain only bytes that are allowed to take on any value, so making them nullable takes extra space.

invalidname25• 3 years ago

This is mentioned in the memory layout section in the post.

crabbone9• 3 years ago

This comes from a misunderstanding of the reason why exceptions where designed they way they were. The whole point of exceptions bubbling up w/o having to write support code to deal with passing exceptions further is to make it so that the purpose of the function is clear to the reader.

Go's exceptions have the same unfortunate property as Java's checked exception. And that's what makes Go's code atrocious. Every other line you see something like:

    if x, e := f(); e != nil {
        ...
    } else {
        return y, e
    }

It makes it very easy to make mistakes when you have to write a lot of repetitive code. You make typos, and because they often land on the "bad" path, they aren't immediately discovered. You have to memorize the state of your function wrt' variable initialization, because now you cannot automatically initialize and destroy them all together. You need to create a lot of helper variables whose purpose is only to transfer return value from one function to another...

If you think that you want checked exceptions, then you don't want exceptions at all. You are denying them the very purpose they were created for. But there are alternative ways to deal with unexpected events in program execution. Monads would be one of those. So... maybe just don't use exceptions?

invalidname25• 3 years ago

No. This comes from a misunderstanding of checked exceptions.

Checked exceptions don't mean I need to handle the exception right now. They mean I need to either do that or declare throws. Declaring throws is fine it implicitly documents the code and enforces a similar requirement up the chain.

Checked exceptions aren't the default and shouldn't be.

dimgl• 3 years ago

Go's exceptions have the same unfortunate property as Java's checked exception.

Wait what? Here's where you lost me. You don't "throw" errors in Golang. Instead it's common practice for functions to return multiple values, one of which can be an error. Errors are just structs that implement the Error interface.

The language (and the compiler by extension) doesn't make you explicitly handle errors. On the contrary: you can choose to ignore errors altogether.

On the other hand, a checked exception in Java is one that MUST be either caught or declared in the method in which it is thrown. Code that fails to do this won't compile. You're completely wrong here.

throwawaymaths• 3 years ago

Not exceptions, but zig has explicit error return types, and it works great. Especially since you can also just "tell the compiler to figure it out". There are a few cases where the compiler can't figure it out, but in those cases you grudgingly annotate the error types and it's done.

breadwinner17• 3 years ago

I have extensive experience in C# as well as Java. It is bizarre to suggest that checked exceptions have failed. It is unchecked exceptions that have failed. It is the biggest flaw of C#, in fact. Why?

As an example, I wrote some very good C# code, carefully tested it, made it work flawlessly, then suddenly it started crashing. What happened? Someone made a change in a function I was calling, and it started throwing a new exception. This would have caused a compile error in Java, not a crash.

The list of recoverable exceptions that can be thrown by a method should be part of the contract. It should be written down by the programmer, and enforced by the compiler. If not, then to avoid crashing you would have to catch the root Exception class, which everyone agrees is a bad idea.

More on checked vs unchecked exceptions here: https://forum.dlang.org/thread/hxhjcchsulqejwxywfbn@forum.dl...

scarface_7413• 3 years ago

Catching the root exception is not a bad idea. If you had caught the new exception what would you have done with it?

Most of the time, it’s either catch the exception, log it in a central logging system keep moving and have a central alerting system or catch the exception log it and crash the program.

revskill• 3 years ago

Catching at the root is only half of story. You must also have ability to resume where it was thrown.

simias4• 3 years ago

Catching the root exception is not a bad idea. If you had caught the new exception what would you have done with it?

Something relevant to the error condition, probably?

There may be some cases where the total set of possible errors is too large to meaningfully handle every one of them specifically (for instance if you call a high level GPU initialization routine that can fail in a myriad of ways) but that's not true in all cases. Maybe the new error was in fact recoverable, or maybe it calls for some more specific diagnostics.

At any rate I completely agree with the parent that changing the set throwable exceptions should be considered a breaking API change and should be enforced by the compiler. If an API method wants to future-proof and be able to throw anything, it can declare just that and everybody will know to expect the unexpected.

That's why I vastly prefer Rust's Result<> system which does most of what exceptions do and with fairly similar ergonomics but with normal return values and all the type checking that it involves.

breadwinner17• 3 years ago

It is a bad idea according to Microsoft.

See https://learn.microsoft.com/en-us/dotnet/fundamentals/code-a...

croes• 3 years ago

you would have to catch the root Exception class, which everyone agrees is a bad idea.

It's a bad idea to catch an unexpected exception?

simias4• 3 years ago

The issue brought up by the parent is that the exception needed not be unexpected. If the compiler had generated an error because a new exception type could be thrown by the third party API, the developer could have decided that they either couldn't do anything with it and just add a catchall, or actually implement code specifically to handle this issue.

IMO the entire concept of "unexpected exception" is bogus in and of itself. That's like calling "exit(1)" in your code when something goes wrong instead of using proper error handling facilities.

Admittedly it's very convenient for small scripts where proper error handling might be overkill, but for any serious application it's a massive footgun IMO.

tom_3• 3 years ago

You run the risk of swallowing something interesting. This was always the problem with C# exceptions to my mind: there was just one mechanism for reporting errors, which covered everything from the most non-erroneous of events, that aren't even errors, such as file not found or invalid file name encoding or failed to write data to file or socket error during write, to stuff that absolutely should cause the program to go pop and die immediately, such as a null pointer or divide by zero.

Anyway, I sympathize with this thread's OP, as I've had exactly the same problem with C#. In the end, I came to the conclusion you kind of do often have to catch every exception, because you can't trust the functions you call, and the documented exception lists for framework stuff are not always accurate. Safest is to put each call in its own try...catch block, and do any property accesses outside the try...catch block so you find out about the NullReferenceExceptions.

This is annoyingly verbose though.

It also does assume your setters and getters don't throw anything.

I mostly liked C#, but the exceptions aspect is not good. Similarly, I've generally despised working with Go, but the way it deals with errors is not its worst feature! (You do have to pay attention to the linter output though! It's the same err variable every time, so the compiler's fastidious checks for variable use are always foiled...)

EMM_386• 3 years ago

Someone made a change in a function I was calling, and it started throwing a new exception.

If it has nothing to do with your code, why not either let it bubble up the stack or try/finally it to do whatever clean-up you need to do before re-throwing it and letting it move on to a global logger or similar.

If it is something you care about you can always catch the specific exception type and handle it.

Maybe whoever worked on the other function wasn't even aware it might throw that particular exception.

Granted I am far more experienced in C# than Java, but the idea that with every code change you potentially have to review and declare every single type of exception seems like madness ("what happens if we pull the cord when it's here, what type will that throw?").

breadwinner17• 3 years ago

Imagine that the functionality implemented by your method is very important. This functionality should not be needlessly aborted. If so you want to be aware of errors you can recover from, correct? This is why checked exceptions are helpful. It gives you a guaranteed-by-compiler list of exceptions and you can decide which of those you should recover from.

beached_whale2• 3 years ago

Why not catch System.Exception also? Or just catch

breadwinner17• 3 years ago

When you catch the root Exception class in C# you end up catching IndexOutOfRangeException as well. You should let the program crash instead because this happened due to a bug in your program. Continuing as if nothing happened is unsafe.

Another issue is that you are not allowing higher layers see the exception, even though they may have logic for recovering from the exception. To see the exception they have to now fix your code by removing the catch-all.

funnymony7• 3 years ago

“I wrote some very good C# code“

We all do.. its always that other guy who is at fault :)

Kwpolska3• 3 years ago

The list of recoverable exceptions that can be thrown by a method should be part of the contract.

If it’s really recoverable, then should it actually be an exception? How often do you see exceptions that are recoverable?

If not, then to avoid crashing you would have to catch the root Exception class, which everyone agrees is a bad idea.

I disagree that it’s a bad idea, having a catch-all exception handler to avoid crashing the entire app is a good idea.

Java also has RuntimeException, and with it, a whole host of exceptions that may occur but are not declared. Should every function that uses the division operator be marked with `throws ArithmeticException` since it may end up being a division by zero? That would quickly become absurd.

kaba05• 3 years ago

Yes, in Java’s parlance an Exception is a recoverable code path, Error is the kind that is not.

E.g. a network call failing due to some IOException can be easily retried, that’s a proper error handling.

chongli• 3 years ago

If it’s really recoverable, then should it actually be an exception?

That's the problem with exceptions in general. They're a hammer that makes everything look like a nail. People start using them for all kinds of control flow situations because they're more convenient than having to deal with a lack of type system support for optional values, etc.

You get to the point where you have a parser that is expecting a digit and it encounters an alphabetic character so it throws an exception!

arzig• 3 years ago

It may not be recoverable locally, but the caller may know how.

Consider Files.createFile. It throws if the target already exists. That method has no way of knowing what the recovery is, however, the caller might.

It could be that the caller wants to explode, but maybe its part of some kind of singleton launch where the recovery is to pass some piece of data off to whoever created the file originally.

Maybe this is a bad example since there’s not much interesting in the return of Files. But it’s exceptional in that it’s an abort rather than a success.

That said, checked exceptions are still trash because they don’t work across thread boundaries or with futures. ExecutionException is the catch all baked into the design because they needed something. The idea could have been good though because naming error handling visible is useful. Java just has the misfortune of designing early before the kinks had been worked out and now we get nice stuff like Rust has.

dbrueck2• 3 years ago

I stopped using Java a long time ago, and so I assume the language has gotten better since then, but early on at least it felt like Java almost took pride in making the developer jump through extra hoops. Compared to many other languages, using Java just made me feel tired.

Checked exceptions - a feature that seems to be a cost to the developer 100% of the time while being a benefit far less than 1% of the time - is a quintessential example of Java's tendency to cause developer fatigue.

breadwinner17• 3 years ago

Some people say the same about strong typing. Like, why do I have to write down the type of every single parameter or variable? Java is making me jump through hoops!

The point is, if you don't need the rigor of a strongly typed compiled language, there are other languages you can use. Perhaps a bash script is all you need.

naasking12• 3 years ago

Checked exceptions - a feature that seems to be a cost to the developer 100% of the time

That's because exceptions are overused for not-truly-exceptional conditions. Languages that throw exceptions when a dictionary does not have a key you're looking for are doing it wrong.

narag• 3 years ago

Java almost took pride in making the developer jump through extra hoops

That's exactly it. A holier-than-thou stance, reprised by most comments in this post.

Checked exceptions were a PITA.

Someone has recommended to use another language if you didn't like it. Unfortunately my employer at that time had entered a partnership with Sun that prevented that :)

Another ideological prohibition: no pointers. Well, how do you assign event handlers then? People were extremely confused with anonymous classes. I got the reason instantly.

Kamq2• 3 years ago

Someone made a change in a function I was calling, and it started throwing a new exception. This would have caused a compile error in Java, not a crash.

I call BS here. I've worked in a few Java projects, and in every single one, the people changing the method in question would have thrown RuntimeException to stop the compile errors.

If RuntimeException was checked, you might have a point, but given that there's a class of unchecked exceptions, you're relying on discipline instead of the compiler anyways.

breadwinner17• 3 years ago

You need to help train your coworkers to write better code :) Point this thread out to them.

Seriously, Java doesn't force bad programmers to write good code. It just enables good programmers to write good code.

crabbone9• 3 years ago

This is very simplistic view of the problem. This completely glosses over modularity, ABI, performance optimizations... just to name a few.

How are you going to write generic functions that take functions as arguments and re-throw the errors thrown by these functions, if you use checked exceptions? Will you require that the acceptable functions only throw exceptions that you like? -- Then your generic function is close to being worthless...

If exceptions are encoded in function's interface, then they have to be in ABI, but then you must have non-trivial types available when marshalling data between two components, so, you cannot serialize the communication using some protocol with a fixed number of types (eg. JSON and friends), because now you need to account for the infinite variety of exception types.

Because of at least these two things, what I saw happen a lot of Java / C++ projects (god blessed me with very little C# exposure) was that as soon as a developer encountered a function with checked exceptions, a wrapper was written which changed the type into a runtime exception. This is so because exceptions are supposed to be handled separately, and often the author of the function has no idea how they need to be handled -- so they want to concentrate on the main goal of the function. Once functions grow into garlands of try-catch-catch-catch-...catch the focus is lost. It becomes very hard to understand why the function was written in the first place, because the error handling takes over every other concern.

duped• 3 years ago

Will you require that the acceptable functions only throw exceptions that you like? -- Then your generic function is close to being worthless...

Constrained generic parameters are actually super useful.

If exceptions are encoded in function's interface, then they have to be in ABI

They already are in Itanium

you cannot serialize the communication using some protocol with a fixed number of types (eg. JSON and friends), because now you need to account for the infinite variety of exception types.

No, the only exception that arises is ser/deserialization error. You can also trivially represent an error in JSON using an object, which is exactly what JSON RPC protocols do. It's also never safe to throw across an FFI boundary and similarly nonsensical to throw across a serialization boundary, so I'm not sure why you'd care.

interlocutor2• 3 years ago

write generic functions that take functions as arguments and re-throw the errors thrown by these functions

There is a philosophy that applies here: simple things should be simple, complex things should be possible. The scenario you're mentioning is not common enough that the language design should be centered around it.

dasyatidprime2• 3 years ago

How are you going to write generic functions that take functions as arguments and re-throw the errors thrown by these functions, if you use checked exceptions?

I've actually done this with an interface type parameter used in a throws clause in Java before, but I'm not sure how it interacts with module boundaries, it's kind of verbose, and it certainly didn't seem to be common practice. But it did work: the HOF-ish method that took a parameterized ThingFrobber<E> and used it to frob things unchecked would compile only if it also threw E, and the method passing the HOF a ThingFrobber (I think via lambda syntax) could catch the associated concrete checked exception type to sink it. IIRC I was using it to allow a checked early exit from a complex iteration, and it even propagated a slightly complex bound with multiple checked exception types more smoothly than I'd expected it to.

osigurdson• 3 years ago

Exceptions are great when they don’t need to be caught. Otherwise they are a pain.

_a_a_a_• 3 years ago

Each 'leaf' statement of language should have a definite and finite list of exceptions that it can throw.

Each non-leaf statement is a composition of leaf statements. Therefore every exception can be determined, and that's our checked exceptions would work; work at the exceptions and check they match the list.

Therefore even without checked exceptions the compiler could provide the exceptions you want and you can check them or not if you please, so get the best of both worlds. All without forcing people to give an explicit list of exceptions everywhere. Forcing checked exceptions is not going to be popular, and much of the time is counter-productive.

Also someone broke your Liskof Substitution Principle which may be your bigger problem.

he00013• 3 years ago

Checked exceptions are great. But they were abandoned due to the functional paradigm so now it’s cumbersome to use them. Errors should be first class of any language and not bolted on as some sort of afterthought, language or design. Errors are something that occurs in real life and should be handled as such. Unchecked errors are a pain to deal with and always are unexpected which you need to dig into the code to see if they are thrown. This is a huge issue for me. I believe that Java should and could make them more easier to use by adding language support to transform them.

masklinn5• 3 years ago

They were abandoned long before “the functional paradigm”. They started being a problem when the Java community decided interactions should go through interfaces over concrete types, and it only got worse with genetics then functional streams.

he00013• 3 years ago

Not sure about that. Reinhold said so explicitly during a conference when they launched Java 8. I don’t see interfaces over concrete types as an issue here.

cubefox5• 3 years ago

While I broadly agree with this, the author doesn't appear to go far enough himself.

Functional error handling, using Option and Result types, is rapidly becoming the standard operating procedure in essentially every language, because it relies on nothing but values and types. They are more of the same, and so they fit right into the existing language machinery.

I agree that Optional types are better than checked exceptions, but they are worse than union types. A function with the union return type String|Error (read: String or Error) is allowed to return another function with return type String. Similarly, a function which accepts the type String|Error as a parameter also accepts parameters of type String. With Option types this doesn't work. Option<String> and String are incompatible types, so code has to be rewritten if the types change.

Personally, I would go so far as to say that union types should replace exceptions in general, checked or unchecked, as well as any implicit nullabiliy, which can be replaced with the explicit union type Foo|Null.

(Although some special syntax for handling Foo|Exception or Foo|Null is probably a good idea, as "error" and "nothing" are pretty general categories.)

Skeime2• 3 years ago

I often run into the opposite problem: There are union types, but I would really like sum types (i.e. tagged unions). The common case is a data structure where I cannot guarantee that the user will not want to use Null as a value in my data structure, but I also need to represent the absence of values myself. With a sum type, my absence would be None and the user’s absence would be Some(None). With union types, I need to somehow create an extra sentinel value myself.

cubefox5• 3 years ago

Something like String|NoAnswer|None doesn't sound too bad to me here...

Sinidir• 3 years ago

Yeah Containers like Option<T> and Result<T> not having a proper subtyping relation to T is major flaw. I mean you are basically giving a stronger guarantee if you are returning T instead of T or an Error, yet you break every callsite.

I have been thinking about this in terms of a data oriented programming language like clojure, but having types/schemas. If you use Union Types for something like getting a value out of a map for a specific key, then if you knew the schema of a specific map you could reduce the return type from {T, Error} to just the type of the value T that you know is there.

Basically a sufficiently smart compiler with the necessary information could make you not have to deal with errors at all in certain cases. With Result/Option/Maybe this would not be possible. It would always infect the entire callstack and you would always have to deal with it.

Kwpolska3• 3 years ago

If your language supports implicit user-defined conversions (like C#), you could `return "foo";` from a method returning `Result<string>`.

Blikkentrekker2• 3 years ago

The problem with union types for exceptions is that it becomes impossible to return an error as the correct case, or at least there is no way to discriminate any more. Functions that deal with errors also need to be able to have exceptional situations, and in many cases there are completely generic functions as well that can error out for generic reasons. A basic example is the elementary case of having a list of errors for some reason and indexing out of bounds producing an error, what if one have a list of values, which are all derived from itself accessing lists, each of which contains “Ok:Val+Err:OutOfBounds”, accessing that list itself now becomes indeterminate.

The other issue is that the language needs to have support for union types rather than merely sum types. And there's a good reason almost no statically typed language has support for union types since it removes provability and it's not something that can in easily be statically decided, requiring dynamic type tags, but then again, that's what sum types effectively are as well.

Typed Racket is statically typed and has union types, but it's an extra layer on top of a dynamically typed language, so the type tags where there already, but it's not easy to erase type data at run time not to implement.

In your example of “String|Error”, it essentially requires that every String is now tagged at runtime with tag information about it in practice.

cubefox5• 3 years ago

And there's a good reason almost no statically typed language has support for union types

For what it's worth, I learned about union types from Ceylon, which was statically typed.

TrianguloY• 3 years ago

As others have mentioned, an interface is a contract and any possible exception that may be thrown needs to be explicitly defined, that's why checked exceptions exists.

However, there is an exception (pun intended) to this: inline lambdas. You can use lambdas as variables, pass them as parameters, etc. And it makes sense for them to retain the interface if needed, but (at least on java) when you are doing a .map(v->parseInt(v)) checked exceptions can't escape the lambda, and that's probably the worst problem of java lambdas.

You can avoid this with custom interfaces where the exception is a generic parameter, but sadly native ones aren't defined like this (yes, exceptions that a function throws can also be defined with generics, and the compiler will automatically use the more restrictive of the lambda code! )

grumbel5• 3 years ago

Little aside, but I feel a lot of the drama surrounding exception could have been solved with a little syntactic sugar making their handling easier. Something along the lines of Perl's "|| die("...")" pattern would be a start (i.e. add some context and rethrow).

In C++ I find it quite infuriating that try{} opens up a new lexical scope, which means you can't construct something, check for errors and move on, since the act of adding a try{} means you'd call the destructor of the thing you just constructed. Meanwhile surrounding a whole block of code with a try{} means you no longer can tell where the exception came from.

senkora• 3 years ago

In C++ I find it quite infuriating that try{} opens up a new lexical scope

That’s an interesting point. I’ve never considered that because I haven’t worked with classes that regularly throw in their constructors.

What situation do you have where 1) you are constructing classes that regularly throw in their constructors and 2) this is a recoverable error that 3) should be handled at the point where the class is constructed instead of some outer point?

grumbel5• 3 years ago

Code like:

  QImage image("filename.png");

is pretty common. Qt solves this by not throwing an exception and using a Null image check:

  bool QImage::isNull() const

but that throws all the extra information that an Exception could provide away.

specialist8• 3 years ago

infuriating that try{} opens up a new lexical scope

This slays me.

The try, catch, and finally blocks should be just one lexical scope.

xen05• 3 years ago

I don't think that helps?

If, as is kind of the point of RAII, your constructors do some initialization and that initialization may fail, then:

If you have a single object that may fail to construct then maintaining it in scope for the catch block doesn't make sense; it's not initialized.

If you have several, then you still can't maintain them in scope, because you don't know which ones are actually valid.

Yes there are other patterns that wouldn't have this issue (acquiring resources outside of the constructor), but then you can just declare your objects outside of the try-catch block.

mostlylurks• 3 years ago

In C++ I find it quite infuriating that try{} opens up a new lexical scope, which means you can't construct something, check for errors and move on

You technically can with a small workaround (demonstrated below), though I personally wouldn't use this approach.

    void foo() {
      auto value = [] {
        try {
          return Foo();
        } catch (...) {
          // Something here
        }
      }();
      value.do_something();
    }

grumbel5• 3 years ago

Yep, I had to resort to that a few times. With a bit of syntactic sugar that could be turned into something like:

    auto value = try Foo("will fail") 
    catch(...) {
        return Foo("will work"); // or throw something else
    };

EricRiese2• 3 years ago

Java has a lot of "throwingness polymorphism". You can extend an interface to say a method throws a subclass of the Exception thrown by the parent interface method. You can extend an interface and say that that method actually doesn't throw an exception at all.

  interface NonThrowingAutoCloseable extends AutoCloseable {
    void close();
  }

You can also declare that you throw a generic exception type

  interface ThrowingSupplier<E extends Exception> {
    void get() throws E;
  }

pshirshov• 3 years ago

The sad truth is: it's pointless to talk about that with those who are not used to structures like Either/Result/IO monads. And those who are already know why and don't need such articles.

tetha2• 3 years ago

Yeah. The thing is, I like the idea of checked exceptions. You could go ahead and define a number of error situations or error kinds and annotate methods with these checked exceptions to force applications to handle these errors. Like, you have a Database.query function and it might throw different exceptions - ConnectionInterrupted, QueryPreparationFailed, QueryFailed, TransactionCancelled. And then you could catch specific error kinds and react differently on those - retry for interrupted connections, cancel on query errors.

This however runs into issues. One really annoying one: Checked exceptions are part of your API. If you forget an error condition and want to introduce a new exception for that, well guess what, that's a breaking API change. There is no easy way to evolve an API with checked exceptions, because changing them requires new major versions, because it prevents code from compiling.

On top, if you want to encapsulate your API properly, you end up with a lot of boilerplate. Once you're using a library, and that library has checked exceptions, you either have to base your own public API on the API of that library - meaning you can never replace it - or you have to start wrapping all manner of errors into your own checked exceptions. I've kinda done it as an experiment some time ago, but that just ends up with so many exception types and so much error wrapping it's kinda ridiculous.

And then you end up with the sad truth on top to be honest: Most error checking is rather brute and clumsy. In most cases, I just let exceptions bubble up because my intermediate function can't really do anything about it. As a distant second, you catch all errors, shove them in some kind of error reporting, reset the system and continue trucking. As a somewhat similar third, I dissect errors in CLI tools to create some useful error messages. And only them I might start caring about some specific errors, but that's pretty rare if you're just running some REST-based business logic.

All in all, it's a good idea, but the implementation results in a lot of API churn or boilerplate for something that's not used much in general.

patrick451• 3 years ago

This however runs into issues. One really annoying one: Checked exceptions are part of your API. If you forget an error condition and want to introduce a new exception for that, well guess what, that's a breaking API change. There is no easy way to evolve an API with checked exceptions, because changing them requires new major versions, because it prevents code from compiling.

This sounds like a good thing. Throwing a new exception is a breaking change whether it's enforced by the compiler or not. Otherwise, callers of you API go from being exception safe to not being exception safe. It's strange to me that you would want to squirm around that just to avoid bumping the version number.

tetha2• 3 years ago

That's not true in java.

You can gradually increase the granularity of unchecked exceptions using subtypes. For example I might initally have AnythingFailedDuringTheQueryException with a Subtype of QueryPreparationException. If I introduce new subclasses of QueryPreparationException to increase the precision of the error reporting, all existing catches for QueryPreparationException will still function as they did before. A "NotEnoughParametersException" subclass of a QueryPreparationError is still caught as a QueryPreparationError. Only if you introduce catches for the new more granular unchecked exception, your codes behavior changes to use the new behavior.

That's a very clean way to improve error reporting in a backwards compatible way.

mkleczek6• 3 years ago

Checked exceptions failed in Java because they don't play well with parametric polymorphism. Union types might help (exception list declaration is actually a union type) but I don't think it would succeed anyway as it is not general enough.

Handling effects and effect polymorphism in programming languages is an active area of research and there are some new languages that try to approach the problem (ie. Koka).

Haskell has several effect libraries (effectful, cleff, eff, polysemy) that look quite nice.

Idris with its dependent types allows precise definition of effects in function signatures.

valenterry2• 3 years ago

No, that's not it. The same problem is true for other error-handling types, such as the "either" or "result" types. Languages without union types or a similar mechanism are unable to define the error types adhoc and force the developer to define them in advance, which is very unergonomic.

But that is true in both cases. The problems with checked exceptions come on top, namely that you cannot use all the regular value-machinery to transform and manipulate the result of a function-call.

mkleczek6• 3 years ago

No, that's not it. The same problem is true for other error-handling types, such as the "either" or "result" types. Languages without union types or a similar mechanism are unable to define the error types adhoc and force the developer to define them in advance, which is very unergonomic

We are saying the same thing, aren't we?

But it is not only about exceptions. Other effects have the same problem and you need a mechanism to define effect polymorphic functions.

hota_mazi• 3 years ago

Checked exceptions failed because developers are lazy and don't want to check for errors.

Checked exceptions force you to do more work, but it's essential work to make your code more robust.

Replacing them with runtime exceptions gives you the illusion of clearer code, but all you have is actually code that is more likely to crash.

jdm22127• 3 years ago

It's not a matter of laziness. Most of the time there's just nothing you can do.

Like if you try to construct a URI based on a configured URI string, and get URISyntaxException... wtf should you even do? You can't recover, because your code shouldn't be mutating config. Similarly with basically any JSON parsing exception. And most disk read/write exceptions.

And in modern applications the caller is probably on the other side of an RPC call, and checked exceptions won't propagate sanely across the RPC anyway.

jdm22127• 3 years ago

Checked exceptions failed because 99 times out of 100 the exception is not recoverable, so the try/catch block is just wasting everyone's time. (In 7 years as a Java dev I can think of one time I wrote code that tried to recover from IOException instead of just making the caller retry.)

Even when the exceptions is theoretically recoverable, it has to get propagated up properly to the caller who should be handling recovery from it. But checked exceptions don't propagate sanely through executors and across RPC calls, so good luck with that.

ratherbefuddled• 3 years ago

Though I mostly agree it's slightly more subtle IMO. They failed because the writer of the method cannot know what the caller can recover from but must decide before compile time. The caller gets little say.

treffer• 3 years ago

I find the rust `?` construct nice for this: give me the success result or propagate the error. It is based on a result type though.

The try/catch construct require too much code for the common propagate case.

It would be nice if Java had a similar construct for error handling.

jayd163• 3 years ago

The Task type in C# with the await unwrap sugar is similar to this. You can even check for and grab the exception without throwing if you want to.

Still, it's too bad the error type must be a throwable. I kind of wish it could just be a plain type so you can error or cancel without generating stack traces. Awaiting a failed task could still throw.

Would be a nice perf boost. As it is now, you don't want to actually cancel or fail a C# task in performance critical code. You need to successfully complete the Task and return an error, which is pretty confusing.

amalcon3• 3 years ago

"throws IOException" is too much code? Or is the issue more that you can't really do autocoercion to a declared thrown type in Java the same way that you can do in Rust?

Proliferation of types is an issue in Java, but the whole language has that problem. It's not just exceptions.

jdm22127• 3 years ago

Yeah, that'd be nice.

jayd163• 3 years ago

Is this a problem with checked exceptions or a problem with Java? Couldn't exceptions be treated as robustly as param and return types in the generic system so they are more composable? Why not a Future<V, E1> interface?

The argument that some exceptions will always be runtime (like oom) so checked exceptions are flawed is harder to argue against but I would also say it's a matter of opinion whether you feel like it's worth dropping entirely.

jdm22127• 3 years ago

You can return union types in Java if you want that, although it'd be nice to have better first class support for them.

The issue though is that forcing people to handle irrecoverable exceptions is just kind of dumb. If I do file IO, I know it'll barf sometimes. Sometimes I care, but the vast majority of the time I don't.

A lot of Java servers have RPC endpoints that basically say "write X to file Y" or "read X from file Y" and if the S3 connection is busted, there's nothing the server can do about that. Someone has to go log spelunking, figure out what's wrong (network is misconfigured, AWS is having an outage, whatever) and fix it. So it is bad API design on Java's part to make every single one of those RPC endpoints wrap every single thing that does file IO in a try/catch.

Gibbon1• 3 years ago

I always feel like the error domain changes as you go across program domains.

I keep returning to something about error handling that bothers me. People argue what the proper way to handle errors is without really considering that it's highly context sensitive. Which makes me think you should be able to pass an error handler down to lower level functions that tells them what to do when something bad happens. Sort of like recent ideas where you pass functions a allocator instead of them calling malloc() directly or whatever.

bluepizza2• 3 years ago

I don't think they are meant to be recoverable from, but instead they are a way to provide a controlled shut down of an irrecoverable failure, or to limit the blast radius of a localized failure.

In that sense, they are quite useful. Saving the file generated an exception - instead of suddenly closing the application, display an error. Or maybe a database is not accessible anymore. Instead of suddenly ending the service, trigger a controlled shutdown (log, send alerts, etc).

I don't think anyone would expect exceptions to propagate through RPC calls. A call fails, probably containing a description of the fail. Why should it propagate?

There are ways to handle errors and return errored RPCs without exceptions, such as internal error codes. But error codes are just as irrecoverable as exceptions.

jdm22127• 3 years ago

In a desktop application that is not allowed to totally crash, or at least has to crash kind of gracefully, checked exceptions are useful.

But in the world that most Java devs live in, which is various flavors of RPC server, failing requests is fine. If lots of requests fail, your monitoring infra should page someone, and that someone will go log spelunking and figure out what's broken.

Very occasionally it turns out that the thing causing the RPC failures is a recoverable exception, and then you should wrap the problematic stuff in a try/catch block. (Often you'll wind up having to detect the recoverable error case by conditioning on substrings in the exception message, which the library owners will arbitrarily change in future releases. So make sure to write regression tests so you'll catch this when you upgrade the library. Java is fun!) But 99% of the time the failure is "network is busted" or "config was invalid" or "hard disk failed" and you should not be defensively programming against all those possibilities.

cvoss• 3 years ago

Can I specify that implementations of foo can throw no, some, or all exceptions? What would it even mean to write something like throws *? Analogously, if I have a function that takes a method as an argument, like a callback, how do I specify what set of exceptions in can throw? Can I have generic “exception set parameters”?
Concretely, checked exceptions in Java failed because Java lacks “throwingness polymorphism”, if you will.

I don't understand. Every "missing feature" the author asks for is, in fact, present in Java. The example below just about sums it up.

  @FunctionalInterface
  interface MyCallback<X, Y, E extends Exception> {

    Y apply (X x) throws E;
    
  }
  
  class A {

    <X, Y, E extends Exception> Y runCallback (MyCallback<X, Y, E> func, X x) throws E {
      return func.apply(x);
    }
    
  }

tadfisher2• 3 years ago

The author's point about the orthogonality of exceptions to values and types still stands; try creating a class like

    abstract class GenericException<R> extends Exception {
        R reason();
    }

As it is, Exception is a "special" type that isn't allowed to be used like normal Java types, because Java generics are a leaky abstraction introduced after checked exceptions, and the JVM cannot deal with type erasure in catch blocks.

aib• 3 years ago

I think the reason is simpler: Unchecked exceptions.

Investing in (checked) exception safety is not as valuable when anything can throw an NPE.

jacquesm• 3 years ago

Erlang does this the right way: the process fails (the Erlang sub-process) but the super visor tree anticipates such failures and will restart the process that failed which if the rest of the application is properly designed will handle the error without loss of consistency. Depending on the failure it can also escalate further up to deal with larger and larger levels of malfunctions. Properly implemented short of gross hardware failure (all nodes down) this will result in a system that is always at or near the maximum possible availability as permitted by the hardware.

smrtinsert• 3 years ago

They didn't fail. I find them useful and great in code reviews where I can see juniors trying to get away with bad practices because they don't feel like making the possible issues known to business owners.

adamwk• 3 years ago

I think Swift’s error model is my favorite (though Go’s new error stuff seems similar). Unrecoverable errors are unrecoverable (they’re aborts) and other errors are marked in the function’s signature and must be handled explicitly. This means the error effect is explicit along the call stack until somebody handles it

Why checked exceptions failed

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