Imperative Object Destruction

Hannah Schroeter [email protected]
Mon, 13 Nov 2000 11:18:55 +0100


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.