http://hackage.haskell.org/package/knob This is a small package which allows you to create memory-backed handles. I found it as a pattern in a few of my test suites, so I spent a day or so polishing it up before posting it to the internet. Feel free to play around with it and tell me about any issues that pop up, or things you wish it could do. The interface is very simple; here's a taste: -------------------------------------------------------------------------- import Data.ByteString (pack) import Data.Knob import System.IO main = do -- for this example, we'll start with an empty knob knob <- newKnob (pack []) -- write to it withFileHandle knob "test.txt" WriteMode $ \h -> do hPutStrLn h "Hello world!" bytes <- Data.Knob.getContents knob putStrLn ("Wrote bytes: " ++ show bytes) -- read from it withFileHandle knob "test.txt" ReadMode $ \h -> do line <- hGetLine h putStrLn ("Got line: " ++ show line) --------------------------------------------------------------------------