def fact x
  if x == 0
    1
  else
    x * fact(x-1)
  end
end

>> fact 10
=> 3628800
>> fact 7
=> 5040

>> fact -1
SystemStackError: stack level too deep
    from (irb):2:in `fact'
    from (irb):5:in `fact'
    from (irb):10

def fact x
  if x < 0
    raise "Can't take the factorial of a negative number"
  elsif x == 0
    1
  else
    x * fact(x-1)
  end
end

>> fact -1
RuntimeError: Can't take the factorial of a negative number
    from (irb):3:in `fact'
    from (irb):10

>> fact 10
=> 3628800
>> fact 7
=> 5040