Hello! On Mon, Nov 13, 2000 at 09:27:07AM -0000, Chris Angus wrote:
why not create an abstract datatype
OpenFile a which is a monad
data OpenFile a = OpenFile (Maybe FileHandle -> IO a)
and create you operations in terms of this
openFile :: String -> OpenFile () count :: OpenFile Int read :: Int -> OpenFile [Byte]
then you could habe a run function
runOF :: OpenFile a -> IO a
which ran whatever you wanted for the file and ensured tahthe file was closed correctly afterwards. How we would do this with 2 files however I'm not so sure.
[...]
That's the problem. And I think your solution is overly complicated. Why not copy what Common Lisp does, just that Haskell can do it without macros: withOpenFile :: FilePath -> IOMode -> (Handle -> IO a) -> IO a withOpenFile name mode action = do handle <- openFile name mode result <- (action handle) `finally` (hClose handle) return result Usage: count :: Handle -> IO Int read :: Handle -> IO [Byte] fileLength <- withOpenFile "filename" ReadMode $ \handle -> count handle fileContent <- withOpenFile "filename" ReadMode $ \handle -> read handle or even, if you want both results without opening the file twice: rewind :: Handle -> IO () (fileLength, fileContent) <- withOpenFile "filename" ReadMode $ \handle -> do len <- count handle rewind handle cont <- read handle return (len, cont) Two files: result <- withOpenFile "file1" ReadMode $ \handle1 -> withOpenFile "file2" ReadMode $ \handle2 -> ... Kind regards, Hannah.