[Haskell-cafe] IO trouble

[Henning Thielemann]

Xiao-Yong Jin schrieb:
> Lauri Alanko <la at iki.fi> writes:
> 
>> On Tue, May 12, 2009 at 04:59:36PM -0400, Xiao-Yong Jin wrote:
>>>> f :: a -> b
>>>> g :: (a -> b) -> c -> d
>>>> gf :: c -> d
>>>> gf = g f
>>> Now I want to handle exceptions in f and redefine f as in f'
>>>
>>>> f' :: a -> IO (Either e b)
>>> So my question is how to define gf' now to use f' instead of
>>> f?
>>>
>>>> gf' :: c -> IO (Either e d)
>> Use Control.Monad.Error.ErrorT, it's exactly for this. You have to
>> "monadize" g to be able to pass f' as an argument to it.
>>
>> f' :: a -> ErrorT e IO b
>> g' :: Monad m => (a -> m b) -> c -> m d
>> gf' :: c -> ErrorT e IO d
>> gf' = g' f'
> 
> So there is no way to do it without "monadize" g to g', is
> it?  Big trouble, sigh.

Sure, but I'm afraid that gets more complicated. Btw. you can also use
ExceptionalT and Exceptional from explicit-exception package, which does
not require a constraint on the exception type.