trees.js

// sums, constructing
const tag = t => {
  function halp() {
    return { tag: t, values: [... arguments] }
  }
  return halp;
};

// testing
tag("label")(1, "horse", [2,3]);

// fruit
const Peach = tag("Peach")();
const Apple = tag("Apple")();
const Pear = tag("Pear")();
const Lemon = tag("Lemon")();
const Fig = tag("Fig")();

// tree
const Bud = tag("Bud")();
const Flat = tag("Flat");
const Split = tag("Split");

const smol_tree = Split(Bud, Flat(Peach, Bud));
const larger_tree = Split(Flat(Apple, Flat(Lemon, Bud)), Flat(Peach, Bud));

// sums, destructing
const match = cases => x => cases[x.tag].apply(null, x.values);

const is_apple =
  match(
    {
      Peach: () => false,
      Apple: () => true,
      Pear: () => false,
      Lemon: () => false,
      Fig: () => false
    }
  );

// testing
is_apple(Apple);
is_apple(Fig);

const height =
  match(
    {
      Bud: () => 0,
      Flat: (_, t) => 1 + height(t),
      Split: (a, b) => 1 + Math.max(height(a), height(b))
    }
  );

// testing
height(smol_tree);
height(larger_tree);