Typed throws in Swift
Swift was promoted as a type-safe programming language on its very first day, and it is solid and safe in many aspects. The part of type safety that needed to be added was throwing functions. Swift 6.0 introduces typed throws, and we will learn all about them this week.
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Let’s start by exploring the straightforward definition of a throwing function in Swift 5.9 or any previous language version.
enum FooError: Error {
case tooBig
case tooSmall
}
func foo() throws -> Int {
let value = Int.random(in: 1...100)
guard value < 60 else {
throw FooError.tooBig
}
guard value > 20 else {
throw FooError.tooSmall
}
return value
}
As you can see in the example above, we define the function that may throw an error while returning an integer value. There is no information on what type of error it may throw. You have to manually scan the function implementation to find all usages of the throw keyword.
func boo() {
do {
let value = try foo()
print(value)
} catch let error as FooError {
switch error {
case .tooBig: print("Too big...")
case .tooSmall: print("Too small...")
}
} catch {
print("General erro: \(error)")
}
}
To handle errors in throwing functions, we have to wrap a call with the do-catch block, cast to the particular error type and handle both type-erased and casted error instances. You must always keep in mind which error type the function may throw because the compiler can’t help you here.
To learn about best practices while modeling error types, take a look at my “Modeling errors in Swift” post.
Fortunately, Swift 6.0 allows us to define the throwing error type explicitly. I say it allows because it doesn’t require defining a type of throwing error.
enum FooError: Error {
case tooBig
case tooSmall
}
func foo() throws(FooError) -> Int {
let value = Int.random(in: 1...100)
guard value < 60 else {
throw .tooBig
}
guard value > 20 else {
throw .tooSmall
}
return value
}
We changed the function definition to provide a throwing error type. Now, we can handle every case of the error type in the catch block. The compiler understands the throwing type and provides us an instance of the particular error type.
func boo() {
do {
let value = try foo()
print(value)
} catch {
switch error {
case .tooBig: print("Too big...")
case .tooSmall: print("Too small...")
}
}
}
Let’s talk more about code compatibility with previous versions of the Swift language. You can still use the old way of defining throwing functions, and it works with Swift 6.0. The compiler translates your code from throwing functions to typed throws.
func foo() throws {
}
// becomes
func foo() throws(any Error) {
}
As you can see, the Swift compiler converts throwing functions to typed throwing using any Error type. Note that even nonthrowing functions translated into throwing where the error type is Never.
func foo() {
}
// becomes
func foo() throws(Never) {
}
Today, we learned how to use new typed throws definition of functions to make our codebase even more predictable and type-safe. I hope you enjoy the post. Feel free to follow me on Twitter and ask your questions related to this post. Thanks for reading, and see you next week!