On Sat, Dec 19, 2009 at 4:46 PM, ntupel
I have looked at the recently released Control.Failure library but I admit, I couldn't understand it completely. So given the example below, how would Control.Failure help me here?
Thanks, nt
-- Theirs (other library code stubs) data TheirError = TheirErrorCase deriving Show data TheirData = TheirData deriving Show
theirFunc :: [String] -> Either TheirError TheirData theirFunc = undefined
-- Mine (my own code stubs) data MyError = MyErrorCase deriving Show data MyData = MyData deriving Show
myFuncA :: TheirData -> Either MyError MyData myFuncA = undefined
-- Ugly. How to apply Control.Failure here? myFuncB :: IO (Either MyError MyData) myFuncB = do let x = theirFunc [] case x of Right x' -> return $ myFuncA x' Left _ -> return . Left $ MyErrorCase _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Well, here's one way of doing it. You have lots of choices here; these are the decisions I made in implementing the code: * myFuncB no longer lives in the IO monad. I wasn't sure if you specifically wanted that, but now it can work with *any* instance of Failure. * Since I assumed you ultimately wanted it to land in the IO monad, I defined Exception instances. However, if you were dealing with a different Failure instance (like [] or Maybe), these would be unncesary. * I also assume that what you meant by "your code" and "their code" is that you can modify your own code, but not theirs. If you show me what the real code is you're working on, I'd be happy to more fully develop a better solution with you. Anyway, here's the code. Michael {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE DeriveDataTypeable #-} {-# LANGUAGE StandaloneDeriving #-} import Control.Failure import Data.Typeable (Typeable) import Control.Exception (Exception) -- Theirs (other library code stubs) data TheirError = TheirErrorCase deriving Show data TheirData = TheirData deriving Show theirFunc :: [String] -> Either TheirError TheirData theirFunc = undefined -- Mine (my own code stubs) data MyError = MyErrorCase deriving (Show, Typeable) instance Exception MyError deriving instance Typeable TheirError instance Exception TheirError data MyData = MyData deriving Show myFuncA :: MonadFailure MyError m => TheirData -> m MyData --myFuncA :: TheirData -> Either MyError MyData myFuncA = undefined myFuncB :: (MonadFailure MyError m, MonadFailure TheirError m) => m MyData myFuncB = do x <- try $ theirFunc [] myFuncA x