[Haskell-beginners] How to make a kinder, gentler "fail"?

[Elvio Toccalino]

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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