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@cdzombak cdzombak/swift2errors.md
Last active Aug 29, 2015

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an executive summary of my ever-forthcoming "error handling in swift 2" blog post. https://twitter.com/cdzombak/status/611704414810832896
  • throws in Swift 2 is just another way to return from a function/method. Because of its propagation semantics, it's not really "exceptions", which is probably a good change.
  • But unlike an Either or Result return type, throws in Swift 2 cannot communicate type information.
  • This sucks; and we will realize it sucks when we keep having to refer to documentation every time we call a function that throws to see what sort of errors we may have to handle. This is part of the problem that types solve; they are documentation, with compiler-enforced guarantees.
  • Arguments based on the clunkiness of pattern-matching on a Result type vs. the new syntax are not valid. Had the Swift team chosen to introduce a blessed stdlib Result type, they could have introduced elegant syntax to handle it without requiring boilerplate; remember, everything related to throws is new syntax.
  • Yes, this would make versioning harder. Changing the types of errors you return changes the contract of an API, if you care about anyone actually handling errors from it. Yes, this sucks for people maintaining APIs. They will have to deal with it.
  • Other counterarguments are based on, effectively, making it slightly harder to write bad code — to ignore errors, catch-all, or simply rethrow errors. But people will go to extraordinary lengths to write bad code. And type-checked exceptions make this no worse; you already have to write a catch-all catch in Swift 2 anyway.
  • Yes, a method that calls a bunch of other methods and rethrows all their errors could have an unwieldy signature. If that is distasteful, do some error handling or coalescing in your method, or just mark it as throwing ErrorType and we're no worse off than we are now.

In closing: tell me what types of errors I have to handle, please. I'd prefer a monadic standard-library-blessed Result type, but throws can work. Just tell me what to catch; tell me what might fail so I have some chance of recovering appropriately.

The following three documents helped me form and solidify my opinions…

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