Swift - is there a way to match an error without throwing? - swift

Question

In some cases, an error is provided as an argument rather than being thrown. One example could be completion handlers where Result<T, Error> is provided. I would like to match the error without throwing it.
Example of error matching by throwing it:
enum MyError: Error {
case error
}
func process(error: Error) {
do {
throw error
} catch MyError.error {
print("this is it")
} catch {
print("unknown error")
}
}
process(error: MyError.error)
UPDATE:
The matching should work even for the system provided errors, like URLError which are not enum types.
func process(error: Error) {
do {
throw error
} catch URLError.timedOut {
print("this is not it")
} catch URLError.cancelled {
print("this is it")
} catch {
print("unknown error")
}
}
process(error: URLError(.cancelled))

Answer 1

You can use the if case syntax.
In your example,
enum MyError: Error {
case error
}
func process(error: Error) {
if case MyError.error = error {
print("this is it")
} else {
print("unknown error")
}
}
process(error: MyError.error)

Answer 2

There are several ways to match errors that are not thrown. A way that keeps the code simple with many errors to check for, is the use of a switch statement:
Update:
I have now updated the code to include the URLError that you have requested.
enum CommonError: Error {
case input
case output
}
enum RareError: Error {
case language
case mathematics
}
let error: Error = RareError.language
// or
let error: Error = URLError(.cancelled)
switch error {
case CommonError.input: print("Common input error")
case CommonError.output: print("Common output error")
case RareError.language: print("Rare language error") // This is executed
case RareError.mathematics: print("Rare mathematics error")
case URLError.cancelled: print("URLError -> cancelled") // - or this
case URLError.timedOut: print("URLError -> timed out")
default: print("The error is unknown; maybe you should consider throwing it now?")
}