> >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
>
> Doesn't fulfill condition 2:
>
> 2. no read or write operations are performed on file-handles
> that have
> not yet been opened, or that have already been closed.
>
> ...since you can do
>
> stealHandle = withOpenFile "filename" ReadMode (\handle -> handle)
> stealHandle >>= read
If you want to enforce this kind of encapsulation using the type system, you
could try using the runST trick, something like:
withOpenFile
:: FilePath -> IOMode
-> (forall s . OpenFile s -> IO a) -> IO a
of course, this needs the universal quantification extensions implemented in
Hugs & GHC.
Cheers,
Simon
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