Error failable

throwable.swift

public extension XCTestCase {
    // We conform to LocalizedError in order to be able to output
    // a nice error message.
    private struct RequireError<T>: LocalizedError {
        let file: StaticString
        let line: UInt
        
        // It's important to implement this property, otherwise we won't
        // get a nice error message in the logs if our tests start to fail.
        var errorDescription: String? {
            return "😱 Required value of type \(T.self) was nil at line \(line) in file \(file)."
        }
    }
    
    // Using file and line lets us automatically capture where
    // the expression took place in our source code.
    public func require<T>(_ expression: @autoclosure () throws -> T? ,
                            file: StaticString = #file,
                            line: UInt = #line) throws -> T {
        guard
            let optionalValue = try? expression(),
            let value = optionalValue else {
            let error = RequireError<T>(file: file, line: line)
            XCTFail(error.errorDescription ?? "", file: file, line: line)
            throw error
        }
        
        return value
    }
}