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

Derrick Ho wh1pch81n at gmail.com
Tue Dec 27 15:03:42 CST 2016

I suppose "throws" could be enhanced to produce a compiletime enum

func bar() throws {
throw NSError()
throw MyEnumError.firstError

Under the hood each expression passed to "throw" could make something like

enum bar_abcdefj_throw {
case genericError // all NSError go here
case MyEnumErrorFirstError

Where abcdefj is unique characters.

This enum would only be visible through the catch blocks which would act
like a switch statement.

Speaking of switch statements do they currently support "try" expressions?

switch try foo() {
case .MyEnumErrorFirstError:
// handle error
// handle NSError and everything else

The benefit of this approach is that it would be additive. And no source

At the risk of being off topic if there is interest in this I can start a
new thread for this.
On Tue, Dec 27, 2016 at 9:54 AM Haravikk <swift-evolution at haravikk.me>

> On 27 Dec 2016, at 13:43, Tino Heth <2th at gmx.de> wrote:
> Imho this is no problem:
> Right now, we basically have "throws Error", and no matter what is
> actually thrown, we can always catch exhaustive by keeping the statement
> unspecific.
> True, but for me it's more about knowing what a method can throw;
> currently to know that you need to consult documentation which, while fine,
> isn't the best way to do that when the compiler could know and it's then up
> to you whether to let it auto-complete for all throwable types, or just be
> generic for some or all of them (or pass the buck).
> myMethod(args:[String:Any]) throws IOError, IllegalArgumentError? { … }
> Imho to much confusion with no real benefit:
> Shouldn't it be up to the caller to decide which errors need special
> treatment? The library can't enforce proper handling at all.
> Partly, but it's really just intended to distinguish things that are
> genuine runtime errors, versus things that shouldn't have happened, i.e- an
> illegal argument type error shouldn't occur if you're using a method
> correctly, so it's more like something you might check as an assertion. I
> should have been more clear that the main difference is in how fix-its
> might recommend the errors are handled; specific types would produce catch
> blocks, but optionals might be grouped into a catch-all at the end (e.g-
> they're special interest that you might not be bothered about), but the
> compiler would still be aware of them should you decide to add them.
> If the distinctions not significant enough though then the "optional"
> error types could just be ignored for now, I think the more important
> ability is the ellipsis indicating "plus other errors" so we can specify
> either exhaustive lists of error types, or keep them open-ended, in which
> case the types listed are those that would be placed as catch blocks, with
> the ellipsis indicating that a catch-all is still required (or throw on the
> current method).
> One thing to note with explicit errors is that we'd basically introduce
> sum types…
> Not necessarily; you could think of explicit errors as being doing
> something like:
> enum MyErrorType {
> case io_error(IOError)
> case illegal_argument(IllegalArgumentError)
> case unknown(Error)
> }
> i.e- boilerplate we could do right now, but would prefer not to, but still
> allowing it to be handled as an enum.
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