gistfile1.rb

# We want a uniform way of reducing a code tree
# so the easiest way to do that is to extend
# Object with a call method and whenever any
# object does not define it's own call method we
# assume the object is implicitly claiming it wants
# to return itself. Since Object is almost at the top
# of the hierarchy almost anything we are interested
# in will work as expected.
class Object
  def call
    self
  end
end

# The children of an instance of a CodeTree consist either
# of more CodeTree instances or just some objects which we
# will pass to the root when it's time to reduce the tree and
# get a value out of it
class CodeTree
  def initialize(proc, *children)
    @proc, @children = proc, children
  end
  
  def call
    @proc.(*@children.map(&:call))
  end
end

plus = ->(a, b) { a + b }
code_tree = CodeTree.new(plus, 1, CodeTree.new(plus, 1, 1))
code_tree.() # ==> 1+(1+1) ==> 3