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.
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.
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.
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.
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.
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)
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".
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").
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
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).
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.
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.
