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' Here "e" should be some fixed instance of Error. HTH. Lauri _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
