Initial data commit

Task/Monads-Writer-monad/Java/monads-writer-monad.java

@@ -0,0 +1,54 @@
+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;
+	
+}