[swift-evolution] throws as returning a Result

Thomas Guthrie tomguthrie at gmail.com
Mon Mar 14 07:15:32 CDT 2016

> On 14 Mar 2016, at 11:36, Ondrej Barina via swift-evolution <swift-evolution at swift.org> wrote:
> it would be great usage of Result. (there was discussion about it. I
> think it was rejected).
> Not really sure about:  "let a: Result<Foo> = makeFoo(42)"
> I can see that this could become more needed/used when we are going
> towards async coding in next Swift (4.0?).

Result is mentioned in https://github.com/apple/swift/blob/master/docs/ErrorHandling.rst#manual-propagation-and-manipulation-of-errors <https://github.com/apple/swift/blob/master/docs/ErrorHandling.rst#manual-propagation-and-manipulation-of-errors> (the proposal/doc for swift’s current error handling) and John McCall had this to say about it:

> We considered it, had some specifics worked out, and then decided to put it on hold.  Part of our reasoning was that it seemed more like an implementation detail of the async / CPS-conversion features we’d like to provide than an independently valuable feature, given that we don’t want to encourage people to write library interfaces using functional-style error handling instead of throws.
> It’s also a feature that’s directly affected by the design of typed throws, which in turn poses some usability challenges for it.  For example, without typed throws you really just want the type to be Result<T>.  With typed throws, can you still write that, or do you have to write Result<T, ErrorType>?  Also, if we want every function result signature to have a corresponding Result<> type, does that permanently prevent us to supporting multiple error types with “typed throws”?  Also, would it be too frustrating to work with typed Result values if we don’t allow implicit covariant conversions along one or both dimensions?

(From https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151207/001433.html <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151207/001433.html>)
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