If you want to use a file in Haskell, you have two options. You can open and close the file yourself:

openFile :: FilePath -> IOMode -> IO Handle
hClose :: Handle -> IO ()

… or you can use a function that takes care of the open-close lifecycle:

withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r

Which is the better interface? It’s a trade-off: withFile guarantees that every open file is closed, while openFile and hClose allow you to open and close files in any order.

Let’s create types to represent these two kinds of interface:

type OpenClose h = IO (h,IO ()) -- opens a resource, returns a handle and a "closer"
type With h = forall r. (h -> IO r) -> IO r

openCloseFile :: FilePath -> IOMode -> OpenClose Handle
openCloseFile path mode = do
    h <- openFile path mode
    return (h, hClose h)
    
withFile :: FilePath -> IOMode -> With Handle
-- already defined

Is it possible to convert between the two? Here’s how to obtain a With interface from an OpenClose interface:

openCloseToWith :: OpenClose h -> With h
openCloseToWith oc f = do
    (h, closer) <- oc
    finally (f h) closer

It’s also possible to obtain an OpenClose interface from a With interface. Can you see how?

withToOpenClose :: With h -> OpenClose h
withToOpenClose = ...?

(answer here)

(first posted here)

— Ashley Yakeley