[Haskell-beginners] How to convert a float or double number into a string?

If I use `show`, show 123.45, it will return "123.45", a desired answer. However, for show 123.45678901234567890, it will return "123.45678901234568". I want to save all digits into a string. I suppose I use a wrong type of number, which is Float. But what can I do to work right? Yi PS: If you receive this mail twice, please ignore one of them.