Thu Feb 24 17:58:36 CET 2011
safeHeadFailure  =3D failure "empty list"
safeHeadFailure (x:xs) =3D return x
safeHead' :: [a] -> Maybe a
safeHead' =3D 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'
add an instance declaration for (Failure String (Either String))
In the expression: safeHeadFailure
In an equation for `safeHead'': safeHead' =3D 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 =3D> [a] -> [b] -> m (a, b)
safeHeadPair'   =3D failure "both lists empty"
safeHeadPair'  _ =3D failure "first list empty"
safeHeadPair' _  =3D failure "second list empty"
safeHeadPair' (x:xs) (y:ys) =3D return (x, y)
safeHead2 :: [a] -> [b] -> Either String (a, b)
safeHead2 =3D 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.
>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=
>> When I write simple parsers and the like I tend to prefer returning usef=
>> strings instead of simply Nothing from Maybe.
>> For example this is a common utility function of mine. Yes, I know with =
>> 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 =3D 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=
>> even work with Maybe since Nothing just drops the string from fail.
>> getNum :: Monad m =3D> String -> m Int
>> getNum n =3D 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 =
>> like to do in my code is define something like
>> class (Monad a) =3D> MonadGentle a where
>> myreturn =3D return
>> myfail s =3D fails s
>> But I can not get an instance of this to compile because it insists myre=
>> myfail are not visible.
>> Since this comes up a lot in the tutorials and books, I am curious why t=
>> not something like MonadGentle in Hackage or the libs. I use mzero occas=
>> but as I said I usually prefer some information with my errors since it =
>> more human usable results.
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