visitor vs catamorphism

gistfile1.rb

class Int
  attr_reader :value
  def initialize(value)
    @value = value
  end
  def accept(visitor, *data)
    visitor.visit_int(self, *data)
  end
  def fold(rules)
    rules[Int].call(value)
  end
end

class Add
  attr_reader :lhs, :rhs
  def initialize(lhs, rhs)
    @lhs, @rhs = lhs, rhs
  end
  def accept(visitor, *data)
    visitor.visit_add(self, *data)
  end
  def fold(rules)
    rules[Add].call(lhs.fold(rules), rhs.fold(rules))
  end
end

class VisitingInterpreter
  def eval(expr)
    expr.accept(self)
  end
  def visit_int(int)
    int.value
  end
  def visit_add(add)
    i0 = add.lhs.accept(self)
    i1 = add.rhs.accept(self)
    i0 + i1
  end
end

class CataInterpreter
  def eval(expr)
    expr.fold(
      Int => lambda { |i| i },
      Add => lambda { |a,b| a + b }
    )
  end
end

Can you please elaborate how CataInterpreter works?