[Haskell-beginners] Writing huge result of `show` to file results in out of memory

I know that there are other ways of doing it, I just want to understand where lies the problem here. I just read the realworldhaskell book by O'Reilly, in one section, Lazy I/O [0], it's explained that there should be no problem writing big strings, without consuming the whole string to files, however big they are. I would appreciate it if someone could point out the problem there. Thanks [0]: http://book.realworldhaskell.org/read/io.html#io.lazy