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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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.
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://www.haskell.org/mailman/listinfo/beginners