[swift-evolution] Type-annotated throws
Bouke Haarsma
bouke at haarsma.eu
Sun Aug 28 03:44:27 CDT 2016
On 2016-08-26 15:39:05 +0000, Félix Cloutier via swift-evolution said:
> Hi all,
>
> Currently, a function that throws is assumed to throw anything. There
> was a proposal draft last December to restrict that. The general idea
> was that you'd write, for instance:
>
>
> enum Foo: ErrorProtocol {
> case bar
> case baz
> }
>
> func frob() throws Foo {
> throw Foo.bar // throw .bar?
> }
>
> If you `catch Foo` (or every case of Foo), now that the compiler can
> verify that your catch is exhaustive, you no longer have to have a
> catch-all block at the end of the sequence.
>
> This impacts the metadata format and has implications on resilience,
> which leads me to believe that the discussion could qualify for the
> phase 1 of Swift 4. If this is the case, I'd be interested in pulling
> out the old discussions and seeing where we left that at.
> Félix
>
> _______________________________________________
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> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
Hi,
Shouldn't the compiler be able to infer the types thrown, and thus
whether all types have been thrown? The general idea being that the
following would be valid:
enum Foo: Error {
case bar
case baz
}
func hello() throws { // inferred to throw only "Foo.bar"
throw Foo.bar
}
func world() throws { // inferred to throw only "Foo.baz"
throw Foo.baz
}
func galaxy() throws { // inferred to throw only "Foo.baz"
do {
try hello()
} catch Foo.bar {
// ...
}
// catch is exhaustive, no catch-all clause needed
try world()
}
func universe() { // all errors are handled, no 'throws' declaration needed
do {
try galaxy()
} catch Foo.baz {
// ..
}
// catch is exhaustive, no catch-all clause needed
}
Now for clarity one could add the type information as per your
proposal, but wouldn't be necessary as the compiler would infer it
itself.
Either way a +1 from me, as the current model forces one to catch-all
errors, even the ones you did not expect to be thrown. Thus potentially
hiding programming errors.
- Bouke
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