[Haskell-cafe] Alternative IO

[Edward Kmett]

Hrmm. This should probably be made consistent with the MonadPlus instance
for IO, so

> empty = ioError (userError "mzero")
Otherwse, I'm surprised this isn't already in the standard library.

I'd suggest submitting it to libraries at .

-Edward Kmett
On Thu, Jul 9, 2009 at 9:27 AM, Cristiano Paris
<cristiano.paris at gmail.com>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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