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

Karl razielim at gmail.com
Tue Dec 27 00:29:04 CST 2016

I agree that this could do with revision. It looks particularly ugly when you’re dealing with throwing closures.

Recently I had a signature like:

public func scan(length: File.ByteOffset.Stride?,
                     by frameSize: File.ByteOffset.Stride,
                     with handler: (_ range: Range<File.ByteOffset>, _ data: UnsafeRawBufferPointer) throws -> Bool) throws -> File.ByteOffset.Stride

It can get difficult to parse the signature towards the end. Marking the handler and function as throwing closer to the beginning would make it more readable IMO. One thing it allows me to do more easily is to remove the spaces around the arrow for the closure parameter (just a little thing which I find helps me read the trailing closure/return type more quickly).

public throwing func scan(length: File.ByteOffset.Stride?,
                                         by frameSize: File.ByteOffset.Stride,
                                         with handler: throwing (_ range: Range<File.ByteOffset>, _ data: UnsafeRawBufferPointer)->Bool) -> File.ByteOffset.Stride

- Karl

> On 26 Dec 2016, at 18:38, 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)
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