[Haskell] [Haskell - I/O] Problem with 'readFile'

[L. J.]

 Hi, I use the operation 'readFile' for obtain information locates on
a file. When I try to write another information on the same file, I
obtain this error message: "openFile: permision denied". I found this:
"The readFile operation holds a semi-closed handle on the file until
the entire contents of the file have been consumed. It follows that an
attempt to write to a file (using writeFile, for example) that was
earlier opened by readFile will usually result in failure with
isAlreadyInUseError." in this web
http://www.haskell.org/onlinereport/io.html.

 How can I break that semi-closed handle for to write in the
preaviously readed file? Thank you.