re-opening a closed stdin?

Simon Marlow
Mon, 25 Nov 2002 12:11:23 -0000

> 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=20
>   mapExceptions :: (Exception -> Exception) -> (a -> a)
> and caught in the IO monad using catch?

No, the report clearly states that they are discarded.

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.