factorial_list_hylomorphism.rb

Pair = Struct.new(:head, :tail)

Either = Struct.new(:val)

class Left < Either
  def value(pair, hylo)
    pair.head
  end
end

class Right < Either
  def value(pair, hylo)
      # Forgive me, Spaghetti Monster, for I have sinned.
    pair.tail.call(val.head, hylo.call(val.tail))
  end
end

list_hylo = -> destruct_f,pair {
  hylo = -> initial { destruct_f.call(initial).value(pair, hylo) }
}

# mult :: Int -> Int -> Int
mult = -> a,b { a * b }

# destruct_count :: Int -> Either () (Int,Int)
#                          LOL
destruct_count = -> n {
  if n == 0
    Left.new( nil )
  else
    Right.new( Pair.new(n, n-1) )
  end
}

fac = list_hylo.call(destruct_count, Pair.new(1, mult))

(0..10).each do |n|
  puts "factorial #{n} = #{fac.call(n)}"
end