import java.util.function.Function;

public final class MonadWriter {

	public static void main(String[] aArgs) {
		Monad<Double> initial = Monad.unit(5.0, "Initial value");
	    Monad<Double> result = initial.bind(MonadWriter::root).bind(MonadWriter::addOne).bind(MonadWriter::half);
	    System.out.println("The Golden Ratio is " + result.getValue() + System.lineSeparator());
	    System.out.println("This was derived as follows:"  + System.lineSeparator() + result.getText());
	}
	
	private static Monad<Double> root(double aD) {
		return Monad.unit(Math.sqrt(aD), "Took square root");
	}

	private static Monad<Double> addOne(double aD) {
		return Monad.unit(aD + 1.0, "Added one");
	}

	private static Monad<Double> half(double aD) {
		return Monad.unit(aD / 2.0, "Divided by two");
	}
	
}
	
final class Monad<T> {		
	
	public static <T> Monad<T> unit(T aValue, String aText) {
		return new Monad<T>(aValue, aText);
	}
	
	public Monad<T> bind(Function<T, Monad<T>> aFunction) {
		Monad<T> monad = aFunction.apply(value);
		monad.text = text + monad.text;
		return monad;
	}
	
	public T getValue() {
		return value;
	}
	
	public String getText() {
		return text;
	}
	
	private Monad(T aValue, String aText) {
		value = aValue;
		text = String.format("%-21s%s%n", "    " + aText, ": " + aValue);
	}
	
	private T value;
	private String text;
	
}