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
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