re-opening a closed stdin?

Dean Herington
Mon, 25 Nov 2002 11:22:56 -0500

Simon Marlow wrote:

> > Simon Marlow:
> > > [Lazy I/O] is nice, but it introduces too many problems.  What
> > > happens to any I/O errors encountered by the lazy I/O?  They have to
> > > be discarded, which means you can't effectively use lazy I/O for
> > > robust applications anyway.
> >
> > Surely they are thrown as exceptions which can then be manipulated
> > in pure code using
> >
> >   mapExceptions :: (Exception -> Exception) -> (a -> a)
> >
> > and caught in the IO monad using catch?
> No, the report clearly states that they are discarded.

Could you please point out where?  I couldn't find it with a quick look.

> We could perhaps have our own versions of the lazy I/O operations which
> throw exceptions, but this in itself is problematic because these kind
> of exceptions would be asynchronous in nature.  If lazy I/O is allowed
> to raise exceptions, then we have a situation where evaluating anything
> can raise an I/O exception.  In theory this shouldn't be a problem - we
> all ought to be writing asynchronous-excpetion-safe code anyway to
> protect against StackOverflow, but an I/O exception is often one that
> you want to handle gracefully and recover from.  I feel distinctly
> uncomfortable about I/O exceptions being thrown by pure code, and even
> more uncomfortable about asynchronous I/O exceptions.

Is even the following example from the library report (section 11.8.2)

import System
import Char( toUpper )

main = do
         [f1,f2] <- getArgs
         s <- readFile f1
         writeFile f2 (map toUpper s)

 -- Dean