[swift-evolution] [Pitch] Typed throws

Karl Wagner razielim at gmail.com
Sat Feb 18 09:27:00 CST 2017

> On 17 Feb 2017, at 19:45, Anton Zhilin via swift-evolution <swift-evolution at swift.org> wrote:
> Now this is on-topic, I guess.
> Last time we stopped at John McCall’s syntax:
> extension MyError: Error { ... }
> func foo() throws(MyError) -> MyResult
> It’s conservative and prevents visual ambiguity with extra parentheses.
> If we (somewhat) agree on this, then submitting a proposal will be trivial.
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So, I’m not sure about what was decided last time, but my issues with this are:

- The thrown error type will become part of the ABI of the function. If you change the type of Error that is thrown, callers may not catch it. At the same time, if we make enums resilient by default and only allow specifying a single entire type, you will basically need one Error enum per function and it will need to be @fixed if you actually want to remove the catch-all block. Otherwise:

// Let’s say this isn’t @fixed...
enum CanFailError {

func canFail() throws(CanFailError) { /* … */ }

do { try canFail() }
catch CanFailError {
    switch error {
        case .errorOne: /* handle error one */
        case .errorTwo: /* handle error two */
        default:        /* handle possible new errors in later versions of the library */

do { try canFail() }
catch .errorOne { /* handle error one */ }
catch .errorTwo { /* handle error two */  }
catch           { /* handle possible new errors in later versions of the library */ }

- I usually have _semantic_ namespaces for Errors, rather than single types per implementation pattern. If we are adding strong annotations about which errors can be thrown, I’d quite like to incorporate that pattern. For example:

extension File {
@fixed enum OpeningError { 
	case .invalidPath
	case .accessDenied  // e.g. asking for write permissions for read-only file
@fixed enum ReadError {
	case .invalidOffset // past EOF
	case .deviceError   // probably worth aborting the entire operation the read is part of

// - throws:
//     - .OpeningError if the file can’t be opened
//     - .ReadError if the read operation fails
func read(from offset: Int, into buffer: UnsafeBufferPointer<UInt8>) throws(OpeningError, ReadError) { /* … */ }

- I wonder if we could try something more ambitious. Since the list of thrown errors is resilience-breaking for the function, it is only beneficial for versioned and @inlineable functions. They should not be able to add new errors (they can remove them though, since errors are intended to be switched over). I wonder if we couldn’t introduce a small pattern grammar for our structured comments (isolated from the rest of the language) - it would be optional, but if you do list your errors, the compiler would validate that you do it exhaustively. Some patterns I would like are:

// - throws: - MyError.{errorOne, errorThree, errorFive}: Something bad      || considered exhaustive
@inlineable public func canFail() throws {}

// - throws: - OpeningError: Computer says nooooo...     || considered exhaustive if OpeningError is versioned or @fixed
//           - *                                         || other errors, requires “catch-all” by external callers
@inlineable public func canFail2() throws {}

If we want to get really clever, we can have the compiler automatically generate those error-lists for internal functions, so you would automatically get exhaustive error-handling within your own module.

- Karl
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