Language feature to improve checked exceptions

[Brian Goetz]

I have some sympathy for the desire to optimize catch-and-wrap; doing 
this the regular way is indeed syntactically painful when the actual 
business logic is small (which it often is.) Catch-and-wrap works well 
as an abstraction idiom when turning a low-level exception (e.g., 
IOException) into a higher-level one (e.g., XMLException); as 
higher-level library-code delegates to lower-level library code, it will 
want to remap low-level errors to higher-level ones.  (Particularly 
interesting is catch-and-wrap at the method declaration level, with some 
sort of `throws X as Y`, as it represents the catch-and-wrap rules 
declaratively rather than imperatively.)

However, the examples you give of catch-and-wrap are not really 
catch-and-wrap; they are catch-and-pretend-they-didn't-happen (by 
turning checked exceptions into unchecked ones.)  While this would 
surely be popular among the "checked exceptions suck" crowd, this is not 
making error handling more reliable, it is just making it easier to 
ignore errors.

The Result approach is both safe and honest (and can be implemented 
today without language help), but is more foreign to Java developers.  
It operates by moving the side-channel result into the main channel, 
which enables monadic composition (turning a partial operation into a 
total one returning a Success|Fail union).  With pattern matching in the 
language, this gets even more attractive.  Still, I suspect that most of 
the "checked exceptions suck" crowd wouldn't thank us for kicking off 
the massive, decade-long migration from checked exceptions to the Either 
monad.

Because of the billions of lines of existing Java code, and the 
almost-billion developer-years of experience with the status quo, This 
problem does not admit easy answers.  We've considered various 
variations of the ideas suggested here, but so far we've not found the 
one we want to commit to for the ages.



On 3/4/2023 6:49 PM, Tom L wrote:
>
> Hello, it's the first time I send an email here, I have no idea how 
> things work, so I hope I am doing things somewhat correctly.
>
> I wanted to make a suggestion about exceptions :
>
> Checked exceptions are used as a meant of a second return type, which 
> shouldn't happen in most cases, and then, depending of what the caller 
> wants to do, it will handle it in a certain way. So in some sort, a 
> checked exception is like part of the return type, like you would have 
> a Result<Success, Failure>, compared to unchecked exception in java 
> which are just supposed to be bugs.
>
> But this feature causes some burden, so much so that some languages, 
> even languages compiling to java (ie kotlin) got rid of checked 
> exceptions, and other languages like Rust use a Result<Success, 
> Failure> which, which, with enough language constructs, can be quite good.
>
> While I agree that not having checked exceptions nor Result but 
> instead only unchecked ones would be a bad idea, because it would mean 
> that a part of the return type is unknown, which we wouldn't want in a 
> strongly typed language, I believe some action needs to be taken.
>
> In my opinion, one of its burdens is caused by the try-catch language 
> feature :
>
> String text;
>
> try {
>
>                 text = Files.readString(somePath);
>
> } catch(IOException ex) {
>
>                 throw new UncheckedIOException(ex);
>
> }
>
> //do something with text
>
> This is a common example, of how you would use it : you want to read a 
> string, and an IO error shouldn't happen, so you fail-fast
>
> The reason why I didn't use text inside the catch, is because the 
> catch should only be for this specific exception, and also it would 
> add another level of nesting, which would start to hurt when other ifs 
> or try-catches appear.
>
> This code is boilerplate and is error-prone, since you always have to 
> repeat the same lines, it is also weird for any non java programmer.
>
> And this is far from being the worst, because another common example 
> is with streams, since you can't throw checked exception, you have to 
> handle each time, in each stream operation, and even if you wrap this 
> code in methods and use method references, it's still far less 
> readable than having a short lambda where you see exactly what the 
> stream is doing.
>
> If checked exceptions were instead a Result type, a solution to this 
> problem would be to make a unwrap() method (like in Rust, or like with 
> Java's Optional#orElseThrow())
>
> String text = Files.readString(somePath).unwrap();
>
> So my suggestion is that, since catching is a language feature, it can 
> only be dealt with another language feature :
>
> String text = Files.readString(somePath) throw IOException ex as new 
> UncheckedIOException(ex);
>
> If this method throws an IOException, it will rethrow it as an 
> UncheckedIOException.
>
> About the syntax, I used "throw" since it's an already used keyword, 
> but depending of the meaning, it could be "catch" or whatever, and 
> "as" could be "->" if using a contextual keyword is too much.
>
> This code could even be simplied as the following with new methods or 
> language features, in the future :
>
> String text = Files.readString(somePath) throw IOException ex as 
> ex.unchecked();
>
> String text = Files.readString(somePath) throw IOException as unchecked;
>
> etc.
>
> Unchecked part could either be the same exception except the compiler 
> ignores it (I know this is possible since it's possible to throw a 
> checked exception as an unchecked), the idea is that since you are 
> telling the compiler that if an exception happens, then it should 
> fail-fast, then the compiler should be able to say "ok, I trust you".
>
> An alternative would be that the unchecked simply wrap it in a 
> RuntimeException or a specific unchecked exception.
>
> So, what's the point of this, does it only serve this use case ?
>
> So first, is it important to note that it isn't just for checked -> 
> fail-fast, but also checked -> some other checked, when exception 
> conversion is needed, which can be useful, and also unchecked -> 
> checked/unchecked, for example if an API only provides an unchecked 
> like Integer#parseInt.
>
> Now about the use case : this syntax, isn't just a compressed 
> try-catch, it's also an expression, which is very important, because 
> not only you can very clearly handle this kind of cases, but it can 
> provide easy to read, concise code in lambdas  and assignments, for 
> example :
>
> try (var files = Files.list(path) {
>
>                 return files.filter(this::matcher)
>
> .map(p -> Files.readString(p) throw IOException as 
> unchecked)//Possible syntax Files::readString throw IOException as 
> unchecked ?
>
> .toList();
>
> }
>
> And in the future, a new method could be added for streams called 
> .continueOnFailure(IOExcepion.class) (or UncheckedIOException if the 
> unchecked wraps the exception instead of marking it as unchecked) for 
> example, which would continue even if there is an exception.
>
> An additional syntax could also be provided for cases where catching 
> is used as an if else :
>
> OptionalInt parseInt(String s) {
>
>                 return OptionalInt.of(Integer.parseInt(s)) catch 
> NumberFormatException -> OptionalInt.empty();            // Using -> 
> syntax here instead of "as" to show that it can also make sense
>
> }
>
> I hope it can fix this unholy war of checked exceptions which never 
> seem to advance.
>
> Sincerely.
>
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