print to standard error in Swift

FileHandleOutputStream.swift

internal struct FileHandleOutputStream: TextOutputStream {
    private let fileHandle: FileHandle
    let encoding: String.Encoding

    init(_ fileHandle: FileHandle, encoding: String.Encoding = .utf8) {
        self.fileHandle = fileHandle
        self.encoding = encoding
    }

    mutating func write(_ string: String) {
        if let data = string.data(using: encoding) {
            fileHandle.write(data)
        }
    }
}
internal var STDERR = FileHandleOutputStream(.standardError)
internal var STDOUT = FileHandleOutputStream(.standardOutput)

print("Hello, world!, to: &STDERR)