Yes, this sounds like what I want but I can not get it to work how I want (-: Monad Reader 15 is where this is discussed. Apparently MonadFailure has been replaced with Failure.
From M.R 15 here is the example: safeHeadFailure :: Failure String m => [a] -> m a safeHeadFailure [] = failure "empty list" safeHeadFailure (x:xs) = return x
safeHead' :: [a] -> Maybe a safeHead' = safeHeadFailure and sure enough, safeHead' works in Maybe. But if I change it to safeHead' :: [a] -> Either String a which is more like what will happen in real code I get: No instance for (Failure String (Either String)) arising from a use of `safeHeadFailure' Possible fix: add an instance declaration for (Failure String (Either String)) In the expression: safeHeadFailure In an equation for `safeHead'': safeHead' = safeHeadFailure Yes, I installed control-monad-failure-mtl and yes I have a 'import Control.Monad.Error' directive in the module. This is straight from the article: safeHeadPair' :: Failure String m => [a] -> [b] -> m (a, b) safeHeadPair' [] [] = failure "both lists empty" safeHeadPair' [] _ = failure "first list empty" safeHeadPair' _ [] = failure "second list empty" safeHeadPair' (x:xs) (y:ys) = return (x, y) safeHead2 :: [a] -> [b] -> Either String (a, b) safeHead2 = safeHeadPair' I get the same failure here. What am I missing? On Thu May 26 11:24 , Elvio Toccalino sent:
Well, there's the 'failure' package. Another package builds on top of it, the control-monad-failure (easier to use, maybe). I read in the monad reader (don't remember the issue number) about it, and have used it ever since. The idea is to let the instanced monad decide what a failure means. Check it out, and comment back.
Cheers.
On Thu, 2011-05-26 at 11:15 -0700, Sean Perry wrote:
This is probably a FAQ of sorts, but I found composing the proper search terms complicated.
When I write simple parsers and the like I tend to prefer returning useful error strings instead of simply Nothing from Maybe.
For example this is a common utility function of mine. Yes, I know with the addition of a Read n qualification I can make this more generic but it does not help with the current discussion.
getNum :: String -> Either String Int getNum n = case reads n of [(x, "")] -> Right x [(x, cs)] -> Left $ "incomplete parse: " ++ cs cs -> Left $ "invalid number: " ++ cs
But I would rather write the following so I am not bound to Either. This would even work with Maybe since Nothing just drops the string from fail.
getNum :: Monad m => String -> m Int getNum n = case reads n of [(x, "")] -> return x [(x, cs)] -> fail $ "incomplete parse: " ++ cs cs -> fail $ "invalid number: " ++ cs
Yeah, I know, no one likes the fact that fail raises an exception. What I would like to do in my code is define something like
class (Monad a) => MonadGentle a where myreturn = return myfail s = fails s
But I can not get an instance of this to compile because it insists myreturn and myfail are not visible.
Since this comes up a lot in the tutorials and books, I am curious why there is not something like MonadGentle in Hackage or the libs. I use mzero occasionally, but as I said I usually prefer some information with my errors since it makes for more human usable results.
Thanks.
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