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> 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.

> Constrained generic parameters are actually super useful.

Then you missed the main point: if you require the generic function to accomodate all sorts of kinds of exceptions raised by the functions it may call, this requirements now propagates to every function it may call. So, you end up implementing functions with unnecessary "throws" because they might be used in a generic function which had to add that clause because of some other completely unrelated function.

Alternatively, you aren't writing generic code at all, you just write two implementations which happen to have similar names.

> 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.

Right, I was thinking that this is the answer to the supposedly intractable problem posed by the original article.

Just make your ExampleInterface generic -- ExampleInterface<T>.foo() throws T.

That technique works really well, and I don’t understand why it isn’t more widely used (or at least it wasn’t when I last used Java for serious work).

> 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.

It's a very common pattern, almost every modern language has a "map" function. The map function can throw a superset of the exceptions that its argument can throw. If checked exceptions can't deal with this, they'll be of limited use.

In the case of map function hopefully you're using it with methods that don't fail in serious ways, and don't need strong error recovery. If so Java has RuntimeException to handle that case. If serious errors are possible and strong error recovery is needed, then you need to avoid the conveniences offered by functional style programming.

The problem is that if some code you call throws a checked exception (InterruptedException being an extremely common culprit) then you must wrap... and suddently nobody calling YOUR code can catch that InterruptedException reliably because it's now a SomeException (doesn't even have to be RuntimeException specifically) with an added "suppressed" exception that you now have to check for.

... so the basic "catch" syntax starts to fall apart because now you have to catch everything and resort to stuff like Guava's Throwables helpers.

It's madness.

The problem ultimately is variance: Methods are covariant, but throws clauses must be contravariant.

There are ways to solve this but "checked exceptions" (as in Java) are not the right way. Ask anyone who's worked in Scala on the JVM which they prefer and you'll have your answer.

It's possible to use a type parameter in a throws clause in Java, last I checked (as I also mentioned in more detail above). But it doesn't seem to be common practice, so interop is still a disaster.

That only works up to one checked exception type, because Java doesn't have anonymous union types.

It really is a lose-lose unless Java grows a more powerful system around this.

Sorry. This is nonsense. map() is the bread and butter of any program. Besides, you cannot ever decide whether an exception is important or not -- it's always in the purview of the user.

Re-throwing a non-checked exception is what I described as the usual / typical coping mechanism in languages with checked exceptions. Which is obviously a way to negate the whole feature.

Sorry, disagree. map() didn't even exist in Java until recently. It is convenience at the cost of some safety. If you're writing a non-critical or throw away code you may not care about strong guarantees. Personally I prefer strong guarantees over convenience. I would only use map() for things that can't throw checked exceptions. A for loop isn't that hard to write.

> The map function can throw a superset of the exceptions that its argument can throw.

Wait. Why should a map care about what exceptions it's arguments can throw?

A map is storing a thingit. A thingit should exist independently before it gets placed into a map. Placing a thingit into a map should not invoke anything on the thingit. The only exceptions coming back from attempting to place a thingit into a map should be exceptions caused by the map.

What am I missing?

Obviously, there are maps that conflate themselves and do things like take ownership when an object is placed into the map. But that's not the general case and presumably you wrote the map specifically with that in mind.

You're thinking of a finite map, also called a hash table or a dictionary.

The map function takes a function and a list and applies the function to every element of the list:

  map(double,[1,2,3]) = [2,4,6]

Grandparent could've used the for loop rather than the map function to make his point:

  for i in [1,2,3]:
      print i * 2

-- because in general instead of the i * 2 we might have a call to a function that might raise an exception.

ADDED. That is wrong: the for loop would not a good example at all because it is not customary to declare the type of a for loop or to need to declare which exceptions a `for` loop might throw.

Whoops. Yeah, I wasn't thinking about map, fold, accumulate, etc. Thanks for the correction.