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bigstrat2003

People bitch about checked exceptions in Java but this is precisely why I think they're a great idea. You can't forget to catch the right type of exception.

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The biggest issue with checked exceptions in modern Java is that even the Java makers themselves have abandoned them. They don't work well with any of the fancy features, like Streams.

Checked Exceptions are nothing but errors as return values plus some syntactic sugar to support the most common response to errors, bubbling.

> They don't work well with any of the fancy features, like Streams.

Because that would require effect types, which is quite advanced/at a research level currently.

All it would require is more support for sum types and variadic type parameters, and maybe fix some hiccups in the existing type inference. You can already write a Stream-like API that supports up to a fixed number of exception types (it’s just a bit annoying to write). The main issue at present is that you can’t do it for an open-ended number of exception types and abstract over the concrete set of types.

The throws clause would require union types, not sum types though (you can observe it in the catch part of a try catch, e.g. `catch ExceptionA | ExceptionB`. But java can't support unions elsewhere, it will have to be replaced by the two exceptions' common supertype.

I was subsuming union types under sum types here, maybe a bit imprecisely.

The following already works in Java (and has for a long time):

    interface F<X extends Exception, Y extends Exception>
    {
        void f(int n) throws X, Y;
    }
        
    void g() throws IOException, SQLException
    {
        F<IOException, SQLException> f = n ->
        {
            if (n > 0) throw new IOException();
            else throw new SQLException();
        };
        
        h(f, 0);
    }
    
    <X extends Exception, Y extends Exception>
    void h(F<X, Y> f, int n) throws X, Y
    {
        f.f(n);
    }

We merely want for F and h to be able to work for any number of exception types. We don't need the ability to declare variables of type X | Y for that.

Of course, it would be nice not having to write IOException, SQLException multiple times in g, and instead have some shortcut for it, but that's not strictly necessary.

The main problem currently is that you have to define F1, F2, F3,... as well as h1, h2, h3,... to cover different numbers of exception types, instead of having just a single definition that would abstract over the number of exception types.

Scala's zio library basically gives you checked exceptions that work with things like type inference, streams, async operations, and everything else.

No, but you can easily end up missing some because somebody wrapped them in some sub-type of RuntimeException because they were forced(!) to. This happens all the time because the variance on throws clauses it at odds with the variance of method signatures (well, implementations, really -- see below).

A new implementation of a ThingDoer usually needs to do something more/different from a StandardThingDoer... and so may need to throw more types of exceptions. So you end up having to wrap exceptions ... but now they don't get caught by, say, catch(IOException exc). If you're lucky you own the ThingDoer interface, but now you have a different problem: It's only JDBCThingDoer which can throw SQLException, so why does code which only uses a StandardThingDoer (via the ThingDoer interface) need to concern itself with SQLException?

Checked exceptions in Java are worse than useless -- they actively make things worse than if there were only unchecked exceptions. (Because they sometimes force the unavoidable wrapping -- which every place where exceptions are caught needs to deal with somehow... which no help from the standard "catch" syntax.)

One thing you can do in Java is parameterise your interface on the exception type. That way, if the implementation finds it needs to handle some random exception, you can expose that through the interface -- e.g. "class JDBCThingDoer implements ThingDoer<SQLException>". Helper classes and functions can work with the generic type, e.g. "<E> ThingDoer<E> thingDoerLoggingWrapper(ThingDoer<E> impl)".

I think this works really well to keep a codebase with checked exceptions tractable. I've always been surprised that I never saw it used very often. Anyone have any experience using that style?

I guess it's not very relevant any more because checked exceptions are sadly out of fashion everywhere. I haven't done any serious Java for a while so I'm not on top of current trends there.

In a former life I worked with a codebase that used that style. Let's just say it isn't enough.

Can you remember what sort of problems you were hitting?

How do you handle the situation where the code might need to throw (pre-existing) exceptions that don't share a useful base class?

I don’t remember! Possibly that’s one of the cases where it doesn’t work out.

Of course, if you had proper sum types, that situation wouldn’t be a problem.

Java has proper sum types, but what one needs here is union types. They are not the same, sum types are labeled and are disjoint.

You want `MyException | ThirdPartyException` here, though.

You're right, I had it backwards! Thanks for the correction.

Now I'm wondering, could that actually be all that's needed to rescue checked exceptions? Is there any language that has that combination of features?

Well, scala has union types, but it doesn't do checked exceptions per say (but it does have a very advanced type system so similar structure can be easily encoded). I think checked exceptions is pretty rare, so I don't know.. probably some research language (but they often go the extra mile towards effect types)

Back when Java didn't have lambdas, one of the more advanced lambda proposals (http://www.javac.info/closures-v06a.html) had this exact thing for this exact reason.

Unfortunately, this take on lambdas was deemed too complicated, and so we got the present system which doesn't really try to deal with this use case at all.

That would be true if not for Java making the critical mistake of excluding RuntimeException from method definitions, so in-practice people just extend RuntimeException to keep their methods looking "clean".

Or are forced to because they want to use generics or lambdas.

Both work with checked exceptions.

The problem is that there's no way to specify an exception specification like "I propagate everything that this lambda throws" (or, for generics, "that method M of class C throws").

No, but you can have an interface like

    interface Func<P, R, X extends Exception>
    {
        R func(P param) throws X;
    }

or the same with more than one exception type, and convert your lambda to that. This works. The only problem is that you can’t abstract over an arbitrary number of exception types.

In principle, one could imagine a syntax for variadic type parameters like

    interface Func<P..., R, X... extends Exception>
    {
        R func(P... params) throws X...;
    }

that would solve that problem.

Will the compiler infer that a lambda or a method ref implements Func with its exception type param, or do you have to rewrite call sites?

The compiler already infers that in current Java for one checked exception type, and also for several exception types at least in some cases (the latter seems to be a little more buggy in the current implementation).

Additional info, they predate Java, having made an appearance in CLU, Modula-3 and C++, before Java was invented.

I miss them in other languages every time I need to track down an unhandled exception in a production server.

>> People bitch about checked exceptions

> they predate Java, having made an appearance in CLU, Modula-3 and C++

Checked exceptions in C++? Can you force/require the call chain to catch an exception in C++? At compile time?

That was part of the idea behind them yes, as many things in WG21 design process, reality worked out differently, and they are no longer part of ISO C++ since C++17.

Although some want to reuse the syntax for value type exceptions, if that proposal ever moves forward, which seems unlikely.

The problem is that a checked exception makes sense only at a relatively high level of the app, but they are used extensively at a low level

My main gripe with checked exceptions is they create a whole other possible code path on each `catch` clause. I tend to keep checked exceptions to the absolute minimum where they actually make sense, all the rest are RuntimeExceptions that should bubble up the stack.

But so would every single other method to react to different types of errors, no?

In something like go, you're even required to create the separate code path for EVERY SINGLE erroring line, even if your intention is simply to bubble it up.

That's kind of how you do it in go. Either:

1. Bubble up error (as is/wrapped/different error. 2. Handle error & have a (possibly complex) new code path.

There's also the panic/recover that sometimes is misused to emulate exceptions.

They don’t create any other code paths than RuntimeExceptions.

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