On: The visitor pattern is essentially the same thing ...

[tiew9Vii]

I use Church encoding quite a lot in Java for a poor mans pattern matching / ADTs. It's one of the more useful patterns I use.

  import java.util.function.Function;

  public abstract class Tree {
      // Constructor private so the type is sealed.
      private Tree() {}

      public abstract <T> T match(Function<Empty, T> a, Function<Leaf, T> b, Function<Node, T> c);

      public static final class Empty extends Tree {
          public <T> T match(Function<Empty, T> a, Function<Leaf, T> b, Function<Node, T> c) {
              return a.apply(this);
          }

          public Empty() {}
      }

      public static final class Leaf extends Tree {
          public final int n;

          public <T> T match(Function<Empty, T> a, Function<Leaf, T> b, Function<Node, T> c) {
              return b.apply(this);
          }

          public Leaf(int n) { this.n = n; }
      }

      public static final class Node extends Tree {
          public final Tree left;
          public final Tree right;

          public <T> T match(Function<Empty, T> a, Function<Leaf, T> b, Function<Node, T> c) {
              return c.apply(this);
          }

          public Node(Tree left, Tree right) {
              this.left = left; this.right = right;
          }
      }
  }

Example usage:

    public String test(Tree t) {
        return t.match(
                empty -> "Empty",
                leaf -> "Leaf",
                node -> "Node"
        );
    }

Original ref: https://apocalisp.wordpress.com/2009/08/21/structural-patter...