On Thu, 9 Jul 2009, Cristiano Paris wrote:
As a joke, I wrote an instance of Alternative for IO actions: {-# LANGUAGE ScopedTypeVariables #-} module Main where
import Control.Applicative import Control.Exception
instance Alternative IO where empty = undefined x <|> y = handle (\ (_ :: SomeException) -> y) x
This would allow to write IO code which failsafes to a value if the previous computation failed, i.e.:
*Main Control.Applicative> undefined <|> print "Hello" "Hello" *Main Control.Applicative> print "Hello" <|> undefined "Hello"
It seems a neat way to catch exception in some scenarios. What do you think? Why is not Alternative IO defined in Control.Applicative?
I just say, what I always say. :-) 'error' denotes a programming error and "catching" it is a hack, sometimes needed but less often than you think. For exceptions one must use 'throw'. Thus, you may e.g. define empty = throw ...