[Haskell-cafe] Re: Exception handling in numeric computations

[Jonathan Cast]

On Thu, 2009-03-26 at 21:57 -0400, wren ng thornton wrote:
> Jonathan Cast wrote:
> > Xiao-Yong Jin wrote:
> > > > Xiao-Yong Jin wrote:
> > > > > So I have another question.  Is the following function safe
> > > > > and legitimate?
> > > > >
> > > > >> safeDiv :: (Exception e, Integral a) =>
> > > > >>            a -> a -> Either e a
> > > > >> safeDiv x y = unsafePerformIO . try . evaluate $ div x y
> > >
> > >> safeDiv' :: (Exception e, Integral a) =>
> > >>             a -> a -> Either e a
> > >> safeDiv' _ 0 = Left e
> > >> safeDiv' x y = Right $ div x y
> > 
> > [...]
> > Other than that, I think the imprecise exceptions paper guarantees that
> > these two functions are equivalent (albeit unwisely: see below).
> 
> I don't think so. The evaluation of x and y may throw errors before we 
> get around to div.

Sure.  Which also points out that the original safeDiv wasn't actually
safe, since there's no guarantee of what evaluate will do with x and y.
(Actually, there's not much guarantee of what evaluate does anyway ---
just that any errors in e's definition get turned into exceptions by the
time evaluate e finishes running, or don't turn into exceptions at all).

> * safeDiv' will evaluate y (to pattern match against 0) and may return 
> an error, e, whereas safeDiv will return Left e if div is strict in y.
> 
> * safeDiv' postpones evaluating x and so may return Right e, whereas 
> safeDiv will return Left e if div is strict in x.

jcc