You should ensure that the result of "evaluate" is in normal form, not just weak
head normal form. You can do this with the Control.Parallel.Strategies module:
import Control.Exception(ArithException(..),try,evaluate)
import Control.Parallel.Strategies(NFData,using,rnf)
import System.IO.Unsafe(unsafePerformIO)
tryArith :: NFData a => a -> Either ArithException a
tryArith = unsafePerformIO . try . evaluate . flip using rnf
test :: [Either ArithException Integer]
test = map (tryArith . (div 5)) [2,1,0,5]
testResult = [Right 2,Right 5,Left DivideByZero,Right 1]
withPair :: Integer -> (Integer,Integer)
withPair x = (x,throw Overflow)
main = do
print (test == testResult)
print (tryArith (withPair 7))
print (tryArith' (withPair 7))
in ghci
*Main> main
main
True
Left arithmetic overflow
Right (7,*** Exception: arithmetic overflow
This "rnf :: Strategy a" ensures that the result of evaluate is in normal form.
This means it should not have any embedded lazy thunks, so any errors from
such thunks will be forced while in the scope of the "try".
Otherwise a complex type like the result of withPair can hide an error.
Xiao-Yong Jin wrote:
Hi,
I just feel it is not comfortable to deal with exceptions
only within IO monad, so I defined
tryArith :: a -> Either ArithException a
tryArith = unsafePerformIO . try . evaluate
and it works quite good as
map (tryArith . (div 5)) [2,1,0,5]
evaluates to
[Right 2,Right 5,Left divide by zero,Right 1]
However, I guess unsafePerformIO definitely has a reason for
its name. As I read through the document in
System.IO.Unsafe, I can't convince myself whether the use of
'tryArith' is indeed safe or unsafe.
I know there have been a lot of discussion around
unsafePerformIO, but I still can't figure it out by myself.
Can someone share some thoughts on this particular use of
unsafePerformIO? Is it safe or not? And why?
Thanks,
Xiao-Yong
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