Moonbase

Logan Capaldo brings us ML-style qualified unions in Ruby:

require 'mltypes'

deftype :List do
  :Cons.of( :first => :Object,
            :rest => :List ) |
  :End
end

deftype :Tree do
  :Leaf.of( :data => :Object ) |
  :Branch.of( :data => :Object,
              :left => :Tree,
              :right => :Tree )
end

def print_list( list )
  case list.tag
  when :Cons
    puts list.first
    print_list( list.rest )

  when :End

  end
end

def print_tree_inorder( tree )
  case tree.tag
  when :Leaf
    puts tree.data

  when :Branch
    print_tree_inorder( tree.left )
    puts tree.data
    print_tree_inorder( tree.right )

  end
end

ls = List.Cons( :first => 1,
                :rest => List.End )
ls = List.Cons( :first => "Hello",
                :rest => ls )
print_list ls

tree = Tree.Branch(
  :data => 2,
  :left => Tree.Leaf( :data => 1 ),
  :right => Tree.Leaf( :data => 3 ) )

print_tree_inorder( tree )

The source for his mltypes library is available in his original post.