Hello, I have a small piece of code that does not compile and I'm having trouble figuring out why. Here is the relevant snippet: readChunks :: Handle -> [String] -> IO [String] readChunks handle accum = do chunk <- readHeaderChunk handle if isLast chunk then return (accum ++ chunk) else return (readChunks handle (accum ++ chunk)) isLast :: [String] -> Bool readHeaderChunk :: Handle -> IO [String] And here is the single compiler error: hacks.hs:48:18: Couldn't match expected type `[String]' with actual type `IO [String]' In the return type of a call of `readChunks' In the first argument of `return', namely `(readChunks handle (accum ++ chunk))' In the expression: return (readChunks handle (accum ++ chunk)) What I'm most confused about is that chunk is passed to isLast as a [String] with no compiler error but cannot be passed to accum ++ chunk that way. Or so it seems. Can someone she some light on this? Thanks.