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. * 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. -- Live well, ~wren