[swift-evolution] Move placement of 'throws' statement

David Sweeris davesweeris at mac.com
Mon Dec 26 13:18:20 CST 2016

> On Dec 26, 2016, at 09:38, thislooksfun via swift-evolution <swift-evolution at swift.org> wrote:
> Hello Swifters,
> I've been writing a lot more Swift code recently, and I have found that the default placement of the 'throws' declaration is often confusing, especially to those of us switching from languages where the type of errors thrown is explicitly defined (like Java)
> For example,
> // This is pretty clear, this can throw an error
> func foo() throws
> { ... }
> // Also pretty clear, this returns a String
> func bar() -> String
> { ... }
> // Confusing. Does this throw a String? Does it return a String? Does it do both?
> // I personally keep reading this as 'this can throw a String'
> func baz() throws -> String
> // Equivalent code in Java (not a model, just for clarification of why the above is confusing)
> String baz() throws StringFormatException
> I therefore suggest either tweaking the syntax around, or moving, the `throws` keyword to avoid this confusion.
> Some ideas I've had:
> // Add a comma to separate them
> func baz() throws, -> String
> // Move `throws` to the end
> func baz() -> String throws
> // Change it to a prefix modifier (like `mutating`)
> throwing func baz() -> String
> I'm still not sold on any of the above syntaxes, but I would love to hear your feedback.
> This would affect existing code, but it would be a fairly small change that would result in very large readability improvements, especially for newcomers, and especially for those coming for a language such as Java.
> -thislooksfun (tlf)

Does `throws` affect the actual return type? That is, is the size of the returned data different between "func foo() -> Int8" and "func foo() throws -> Int8"? If so, the "throws" is quite literally part of the return type and the current syntax reflects that. If not, I think I'd probably be in favor of that last "prefix modifier" suggestion with either "throwing" or "@throwing" (depending on the exact semantics of the "@" part — I'm a bit unclear on that detail)... probably... maybe... I'll have to think about it some more.

- Dave Sweeris

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