On Fri, Mar 15, 2013 at 5:52 AM, Jacek Dudek
readFile fileName >>= \ contents -> writeFile fileName (f contents)
You're reading and writing to the /same/ file back to back. With Lazy I/O. Those two just don't mix. The "permission denied" probably stems from the attempted write clashing with the previous exclusive read handle.
I thought lazy IO was implemented in such a way that you were safe to INTERPRET the IO action as having been fully performed.
Nope. The usual gotcha lying for the unwary is do h <- openFile fname1 ReadMode s <- hGetContents h hClose h writeFile fname2 s In the case fname1==fname2, a different surprise lies in store as you've just witnessed. Importing System.IO.Strict from the strict package should fix all of the above. -- Kim-Ee