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 ...
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