[Haskell-cafe] Alternative IO

[Thomas Davie]

To be honest -- that seems rather nice.  Can has in Hackage?

Bob

On 9 Jul 2009, at 15:27, 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?
>
> Thanks,
>
> Cristiano
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