Re: [Haskell-cafe] Is there a generic way to detect "mzero"?

Hi Antoine and Tobias (and everyone else), Thanks a lot for your answers. They are really helpful Can you please show me how to use the (Eq m) constraint to do this? Also, my general question (probably novice-level) is that in monadic programming, you can convert not necessarily monadic codes into monadic ones. I know for many cases, it is impossible to do the reverse conversion, e.g. you can't make a function involving real IO operations into a pure code. In other cases, for example, I may need to using things like Nothing as the "null" value as in other programming languages, just to represent a special "missing" value outside the regular type. Is mzero a reasonable replacement for this or is there any reasonable (abstract) approximation in Haskell for doing this? (Like "null", I need the ability to detect it.) Thanks, Ting

From: aslatter@gmail.com Date: Mon, 26 Mar 2012 14:24:09 -0500 Subject: Re: [Haskell-cafe] Is there a generic way to detect "mzero"? To: tinlyx@hotmail.com CC: haskell-cafe@haskell.org

On Mon, Mar 26, 2012 at 1:33 PM, Ting Lei wrote:

...

Hi,

I was writing a code trying to use MonadPlus to detect some error cases (representing missing values etc. in pure code). With the Maybe monad, I can do this:

can0 :: (a -> Maybe b) -> a -> Bool can0 f x = case f x of Nothing -> False Just x -> True

And I got the expected result:

*Main> can0 (\x -> Just x) 1 True

But, when I try to generalize this using MonadPlus, as follows:

can :: (MonadPlus m) => (a -> m b) -> a -> Bool can f x = case f x of mzero -> False _ -> True

I got a warning:

__testError.hs:31:11: Warning: Pattern match(es) are overlapped In a case alternative: _ -> ... Ok, modules loaded: Main.

Well, you can sort of do it with only MonadPlus - but it really depends on your choice of Monad whether or not it does anything like what you want:

can :: (MonadPlus m) => (a -> m ()) -> a -> m Bool can f x = (f x >> return True) <|> return false

For 'Maybe' this works great, but for something like 'List' I couldn't even tell you what it would do without reasoning through it.

So you might be better off with the suggestion from Tobias using Eq

Antoine