> On Feb 27, 2017, at 2:08 PM, Matthew Johnson <[email protected]> wrote:
>
>>
>> On Feb 27, 2017, at 1:46 PM, David Waite via swift-evolution
>> <[email protected] <mailto:[email protected]>> wrote:
>>
>> Add more layers, and it can be very mysterious why a call failed. Java at
>> least captures stack traces in this case to aid in technical support in
>> diagnosing the error.
>>
>> Wrapping exceptions also prevents an aggregate of errors from different
>> subsystems being handled as a category, such as having a catch block handle
>> RecoverableError generically
>>
>> An interesting solution that has emerged in Ruby to keep library authors
>> from wrapping exceptions is by decorating the existing exception. Exceptions
>> are caught via pattern matching (same as in Swift), so rather than wrap an
>> extension, they extend the error instance with a library-specific module
>> (e.g. swift protocol). So while the error may be a IOError in ruby, you can
>> still catch it via ‘rescue JSONError’
>
> If I understand this correctly it sounds like introducing a library would
> create protocols to categorize errors and add retroactive conformances to
> these protocols for errors thrown by its dependencies? That is an
> interesting approach. But it requires knowing the concrete types of the
> possible errors all the way down the stack (you can’t add a conformance to an
> existential). This seems very problematic to me, especially in a language
> where creating new error types is as easy as it is in Swift.
I believe it is something that even Objective C can’t do; extend a single
instance of a type to support a protocol.
You can still do really interesting things with categories of errors, such as
extend existing types to support a protocol, and putting categorization logic
on third party errors yourself.
That breaks though if every library does
struct MyError: Error {
let innerError: Error
}
in order to have a typed throw.
> Error handling is messy, there’s no doubt about that. I would like to have
> as many tools at my disposal as possible. Error types is one of those tools.
I’m still trying to mentally put together the elevator pitch for typed throws.
How do I explain to new/junior developers and developers coming from other
languages when to do typed throws, and when not to?
-DW
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