Read and Write to Same File

I am writing a program that reads and writes to the same file. I was having some problems with it writing to the file before it read it. I solved it in the following two ways: 1. main = do text <- readFile "test" let something = somefunc text writeFile "test2" something renameFile "test2" "test" 2. main = do text <- readFile "test" let something = somefunc text writeFile "test" $! something Are both of these correct (guaranteed to give the behavior I want)? Which is better (and why)? Thanks!