Simple test around throwing close function and api design

macOSplayground.swift

enum ConnectionError: Error {
    case fakeWork
    case close
    case connect
}

struct Connection {
    func close(fail: Bool = false) throws {
        if fail {
            print("throwing during close connection ")
            throw ConnectionError.close
        }
        print("closing connection")
    }
    
    static func connect(fail: Bool = false) throws -> Connection {
        if fail {
            print("throwing during open connection")
            throw ConnectionError.connect
        }
        print("opened connection")
        return Connection()
    }
}

func doWork(fail: Bool) throws {
    if fail {
        print("throwing during doWork")
        throw ConnectionError.fakeWork
    }
    print("work done successfully")
}



func test() throws {
    let connection = try Connection.connect(fail: false)
    
    defer {
        do {
            try connection.close(fail: true)
        } catch {
            // Can't throw from a defer
            // if doWork succeeds but close fails then we'd like to throw the close error...
            // otherwise close shouldn't be a throwing function.
        }
    }
    try doWork(fail: true)
}


// Have to write it like this, unless there's a better way?
// play with booleans passed to connect(), doWork(), and close() (two spots)
// to explore behavior.
func betterTest() throws {
    let connection = try Connection.connect(fail: false)
    
    do {
        try doWork(fail: true)
        try connection.close(fail: false)
        print("success!")
    }
    catch ConnectionError.close {
        throw ConnectionError.close
    }
    catch {
        do {
            try connection.close(fail: true)
        } catch {
            print("Silencing error thrown by close after previous errors")
        }
        throw error
    }
}

// testing:
print("doing test()\n")
do {
    try test()
    print("success!")
} catch {
    print( "test threw: \(error)")
}
print("\ndone test()-ing\n\n---\n\n")


print("Doing betterTest\n")
do {
    try betterTest()
} catch {
    print("betterTest threw: \(error)")
}
print("\nDone with betterTest\n")

Output with boolean parameters as in gist (play with them to explore code):

doing test()

opened connection
throwing during doWork
throwing during close connection
test threw: fakeWork

done test()-ing

---

Doing betterTest

opened connection
throwing during doWork
throwing during close connection
Silencing error thrown by close after previous errors
betterTest threw: fakeWork

Done with betterTest