Hi Dan,
io_hash_tuples <- map do_prefix_hash filenames
You're operating inside of the IO monad, so anything on the right hand side of the `<-` has to have the type `IO`. If you're looking up the type of `map`, you will see that it doesn't return the right type. Most likely you just wanted to create a binding like: let io_hash_tuples = map do_prefix_hash filenames
hash_tuples <- sequence io_hash_tuples
`io_hash_tuples` is of type `[(IO String, String)]`, but `sequence` expects a `[IO a]`. Looking at your code, it's easier not to put the `IO String` computation of the hash into a tuple, but first compute all hashes: hashes <- sequence (map Md5s.prefix_md5 filenames) And if you want the hash and the filename grouped in a tuple: zip filenames hashes Greetings, Daniel