Throwing inside a guard

throw_test.swift

/// Lesson learned:
/// Inside a throwing function...
/// if a `guard` condition calls a second function that ends up throwing...
/// the second function's error is immediately propagated outside of the function.
/// Execution does not enter the `else` block of the `guard` clause.

struct PlaygroundError: Error {
    let message: String
}

enum DesiredAction {
    case throwSomething
    case returnNil
    case returnString
}

func performAction(_ action: DesiredAction) throws -> String? {
    switch action {
    case .throwSomething:
        throw PlaygroundError(message: "I'm throwing from inside the method")
    case .returnNil:
        return nil
    case .returnString:
        return "Here's a valid string for you"
    }
}

func callingFunction_throw() throws -> String {
    guard let stringValue = try performAction(.throwSomething) else {
        throw PlaygroundError(message: "This is an error from the calling function, which I don't want to see")
    }
    return stringValue
}
func callingFunction_returnNil() throws -> String {
    guard let stringValue = try performAction(.returnNil) else {
        throw PlaygroundError(message: "This is an error from the calling function, which I don't want to see")
    }
    return stringValue
}
func callingFunction_returnString() throws -> String {
    guard let stringValue = try performAction(.returnString) else {
        throw PlaygroundError(message: "This is an error from the calling function, which I don't want to see")
    }
    return stringValue
}

print("1: return string")
do {
    try print(callingFunction_returnString())
} catch let error {
    print("Error:", error)
}
print("2: return nil")
do {
    try print(callingFunction_returnNil())
} catch let error {
    print("Error:", error)
}
print("3: throw")
do {
    try print(callingFunction_throw())
} catch let error {
    print("Error:", error)
}

/*
1: return string
Here's a valid string for you
2: return nil
Error: PlaygroundError(message: "This is an error from the calling function, which I don\'t want to see")
3: throw
Error: PlaygroundError(message: "I\'m throwing from inside the method")
*/

Also, I learned that these produce the same output:

try print(callingFunction_returnString())
print(try callingFunction_returnString())