add Either class

src/main/java/casser/support/Either.java

@@ -0,0 +1,59 @@
+package casser.support;
+
+import java.util.function.Function;
+
+public final class Either<L, R> {
+
+	private final L left;
+	private final R right;
+	
+	private Either(L left, R right) {
+		this.left = left;
+		this.right = right;
+	}
+	
+	public boolean isLeft() {
+		return left != null;
+	}
+	
+	public L getLeft() {
+		return left;
+	}
+
+	public boolean isRight() {
+		return right != null;
+	}
+
+	public R getRight() {
+		return right;
+	}
+
+	public static <L, R> Either<L, R> left(L left) {
+		return new Either<L, R>(left, null);
+	}
+
+	public static <L, R> Either<L, R> right(R right) {
+		return new Either<L, R>(null, right);
+	}
+
+	public EitherCase getCase() {
+		if (left != null) {
+			return EitherCase.LEFT;
+		}
+		else if (right != null) {
+			return EitherCase.RIGHT;
+		}
+		throw new IllegalStateException("unexpected state");
+	}
+	
+	public <T> T fold(Function<L, T> leftFunction, Function<R,T> rightFunction) {
+		switch(getCase()) {
+		case LEFT:
+			return leftFunction.apply(left);
+		case RIGHT:
+			return rightFunction.apply(right);
+		}
+		throw new IllegalStateException("unexpected state");
+	}
+	
+}

src/main/java/casser/support/EitherCase.java

@@ -0,0 +1,5 @@
+package casser.support;
+
+public enum EitherCase {
+	LEFT, RIGHT;
+}