パターンマッチで fold してみた

fold.rb

def fold(init, list, &block)
  case list
  in []
    init
  in [x, *xs]
    fold(block.call(init, x), xs, &block)
  else
    raise ArgumentError.new("Array is expected but #{list.inspect} is given")
  end
end

def sum(list)
  fold(0, list, &:+)
end

puts sum [1,2,3,4,5] #=> 15