Am Donnerstag, 9. Juli 2009 15:27 schrieb Cristiano Paris:
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?
Thanks,
Cristiano
Hello Cristiano, I fear that this instance doesn’t satisfy required laws. As far as I know, the following equalities should hold: (*>) = (>>) f *> empty = empty empty <|> g = g This implies the following: (f >> empty) <|> g = g But this wouldn’t hold with your instance. (f >> empty) <|> g would cause the side effects of f and of g, while g would (obviously) only cause the side effects of g. If empty would be a real empty, it would have to undo the effects of previous actions (like f above). So an Applicative instance makes sense for STM but not for IO. Best wishes, Wolfgang