[swift-evolution] Throws? and throws!

[Joe Groff]

> On Jan 12, 2017, at 2:58 PM, Jonathan Hull via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I really like swift’s error handling system overall. It strikes a good balance between safety and usability.
> 
> There are some cases where it would be nice to throw errors, but errors are rarely expected in most use cases, so the overhead of ‘try’, etc… would make things unusable. Thus fatalError or optionals are used instead.  For example, operators like ‘+’ could never throw because adding ’try’ everywhere would make arithmetic unbearable. But in a few cases it would make my algorithm much cleaner if I just assume it will work and then catch overflow/underflow errors if they happen, and resolve each of them with special cases.  Or perhaps I am dealing with user entered values, and want to stop the calculation and display a user visible error (e.g. a symbol in a spreadsheet cell) instead of crashing.
> 
> I would like to propose adding ‘throws?’ and ‘throws!’ variants to ‘throws’.
> 
> These would be used for cases where error handling is not the default desired behavior, but having it as an option is desired occasionally.  Essentially, the user would no longer have to preface the call with ‘try’, as the compiler would implicitly add ‘try?’ or ‘try!’ respectively.
> 
> Thus, the function would act like a non-throwing function (either trapping or returning an optional in the case of error), but the user could add ‘try’ to the call to override that behavior and deal with the error more explicitly.
> 
> Another example would be bounds checking on arrays.  If subscripting arrays was marked as ‘throws!’ then it would have the same default behavior it does now (trapping on bounds error).  But a user could add ‘try?’ to return nil for a bounds error in cases where they explicitly want that, or they could add ‘try’ to deal with it as an error using do-catch.
> 
> I think this would really increase the availability of error handling in areas where it is impractical right now…
> 
> Thanks,
> Jon
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Another place I can see this being useful is for higher-order functions which "obviously" work on a throwing operation. The most obvious example would be a constructor for a Result type that collects the success or error result from a throwing closure:

enum Result<T> {
  case ok(T)
  case error(Error)

  init(_ f: () throws -> T) {
    do {
      self = .ok(try f())
    } catch {
      self = .error(error)
    }
  }
}

Having to write `Result { try somethingThatMightFail() }` feels a bit silly because "obviously" the operation you pass to Result is going to be able to fail. Such a feature does feel like it could be easy to misuse, though.

-Joe