Xiao-Yong Jin schrieb:

Lauri Alanko writes:

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On Tue, May 12, 2009 at 04:59:36PM -0400, Xiao-Yong Jin wrote:

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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'

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f' :: a -> IO (Either e b) So my question is how to define gf' now to use f' instead of f?

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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.