Algebrick

It's a gem providing algebraic types and pattern matching seamlessly integrates with standard features Ruby.

Documentation: http://blog.pitr.ch/algebrick
Source: https://github.com/pitr-ch/algebrick
Blog: http://blog.pitr.ch/tag/algebrick.html

What is it good for?

Defining data structures.
Algebraic types play nice with JSON serialization and deserialization. It is ideal for defining message-based cross-process communication.
and more...

Quick example

Let's define a Tree

Tree = Algebrick.type do |tree|
  variants Empty = atom,
           Leaf  = type { fields Integer },
           Node  = type { fields tree, tree }
end

Now types Tree(Empty | Leaf | Node), Empty, Leaf(Integer) and Node(Tree, Tree) are defined. Let's add a method, don't miss the pattern matching example.

module Tree
  # compute depth of a tree
  def depth
    match self,
          (on Empty, 0),
          (on Leaf, 1),
          # ~ will store and pass matched parts to variables left and right
          (on Node.(~any, ~any) do |left, right|
            1 + [left.depth, right.depth].max
          end)
  end
end

Method defined in module Tree are passed down to all values of type Tree

Empty.depth                                        # => 0
Leaf[10].depth                                     # => 1
Node[Leaf[4], Empty].depth                         # => 2
Node[Empty, Node[Leaf[1], Empty]].depth            # => 3