[swift-evolution] [Pitch] Typed throws

David Waite david at alkaline-solutions.com
Mon Feb 27 13:46:41 CST 2017

IMHO, there are two kinds of responses to errors - a specific response, and a general one. Only the calling code knows how it will deal with errors, so a “typed throws” is the function guessing possible calling code behavior.

The problem is, that gives four possible combinations - two where the function guesses correctly, and two where it doesn’t. The most damaging is when it suspects a caller doesn’t care about the error, when the caller actually does. This is unwanted wrapping.

To provide an example, imagine a library that parses JSON. It has several errors indicating JSON syntactic errors, and an “other” for representing errors on the input stream. It wraps the input stream errors so that it can provide a closed set of errors to the caller.

The caller is responsible for returning a data set. It doesn’t think that code calling ‘it” cares about JSON syntactic errors, merely that the object was not able to be restored. It returns its own wrapped error.

However, the original caller knows it is loading from disk. If the problem is due to an issue such as access permissions, It has to know implementation details of the API it called if it wishes to dive through the wrapped errors to find out if the problem was filesystem related.

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’

Trying to specify the exact errors becomes even more destructive with protocols and closures, where the person defining the interface knows neither which errors the implementor of the call will throw, nor necessarily if the caller will want to implement specific behavior on those errors. This in my personal Java coding experience almost always leads to wrapping in some protocol-specific Exception type which exposes minimal information to the caller, or exposing your errors in some unrelated type like IOException which was declared based on the author’s experience of possible exceptions.

> On Feb 27, 2017, at 5:19 AM, Daniel Leping via swift-evolution <swift-evolution at swift.org> wrote:
> On Mon, 27 Feb 2017 at 8:44 Dave Abrahams via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> on Fri Feb 17 2017, Joe Groff <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> > Experience in other languages like Rust and Haskell that use
> > Result-based error propagation suggests that a single error type is
> > adequate, and beneficial in many ways.
> And experience in still others, like C++ and Java, suggests that
> using static types to restrict the kind of information a function can
> give you when an error occurs may actually be harmful.
> +1 here. It becomes wrapping over wrapping over wrapping. Try doing a big app in Java (i.e. some kind of layered server) and you'll understand everything. Ones who tried and still want it - well, there are different tastes out there.
> --
> -Dave
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