On 15-Jun-1998, Fergus Henderson <[EMAIL PROTECTED]> wrote:
> On 12-Jun-1998, Scott Turner <[EMAIL PROTECTED]> wrote:
> > At 14:40 1998-06-10 +0100, you wrote:
> >
> > >Here's a reasonable design for exceptions in Haskell:
> >
> > >* handle :: (String -> IO a) -> IO a -> IO a
> >
> > You probably realized more quickly than I how this
> > can leak exceptions.
> ...
> > Is this considered a drawback?
>
> This kind of exception handling can "leak" exceptions, but not in the
> way you described.
...
> > What I mean is
> >
> > main = do quotient <- handle (const (return 0)) (return (0 / 0)
> > -- Looks plausible
> > -- but the exception isn't raised yet.
> > print quotient -- Here the expression 0/0 is evaluated
> > -- and the exception is raised with no handler.
>
> This is not correct. This example would print out `0' rather than raising
> an uncaught division by zero exception.
I'm afraid I must retract those statements. Scott Turner was quite correct,
and I was mistaken. My apologies!
As Scott pointed out to me in personal email, SLPJ's definition of `handle'
> | * handle :: (String -> IO a) -> IO a -> IO a
> | (handle h a) tries to perform the action a.
> | If doing so delivers a set of exceptional values then
> | apply the exception handler h to the string that names
> | one of them. It is not defined which of the exceptional
> | values is picked.
means that it only catches exceptional values in the I/O action,
not exceptional values in the return value.
Regarding Scott's question
> Is this considered a drawback?
my answer is still much the same -- yes, it's a drawback,
but I'd place the blame more on laziness than exception handling.
I consider it only a minor drawback, since the "leakage" can be
avoided if you use a version of `handle' which is strict in the return
value, e.g.
strict_handle handler action = handle handler strict_action where
strict_action = do value <- action
seq value return value
-- or with `hyper_seq' instead of `seq'
--
Fergus Henderson <[EMAIL PROTECTED]> | "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh> | of excellence is a lethal habit"
PGP: finger [EMAIL PROTECTED] | -- the last words of T. S. Garp.
