Simplest way to throw an error/exception with a custom message in Swift?

Question

I want to do something in Swift that I'm used to doing in multiple other languages: throw a runtime exception with a custom message. For example (in Java):

throw new RuntimeException("A custom message here")

I understand that I can throw enum types that conform to the ErrorType protocol, but I don't want to have to define enums for every type of error I throw. Ideally, I'd like to be able mimic the example above as closely as possible. I looked into creating a custom class that implements the ErrorType protocol, but I can't even figure out that what that protocol requires. Ideas?

Answer 1

The simplest approach is probably to define one custom enum with just one case that has a String attached to it:

enum MyError: ErrorType {
    case runtimeError(String)
}

Or, as of Swift 4:

enum MyError: Error {
    case runtimeError(String)
}

Example usage would be something like:

func someFunction() throws {
    throw MyError.runtimeError("some message")
}
do {
    try someFunction()
} catch MyError.runtimeError(let errorMessage) {
    print(errorMessage)
}

If you wish to use existing Error types, the most general one would be an NSError, and you could make a factory method to create and throw one with a custom message.

Answer 2

The simplest way is to make String conform to Error:

extension String: Error {}

Then you can just throw a string:

throw "Some Error"

To make the string itself be the localizedString of the error you can instead extend LocalizedError:

extension String: LocalizedError {
    public var errorDescription: String? { return self }
}