Re: [Haskell-cafe] golf, predicate check function for MonadPlus (was Re: How to read safely?)

On Thu, Jul 2, 2009 at 3:36 AM, Jon Fairbairn wrote:

Dan Doel writes:

...

There was talk of adding a readMaybe a while ago, but apparently it never happened.

As it is, you can use reads, "read s" becomes:

    case reads s of       [(a, rest)] | all isSpace rest -> <code using a>       _                              -> <error case>

which ensures that you have an unambiguous parse with only trailing whitespace. You can, of course, modify that if you don't care about ambiguity or trailing characters.

I was wondering about a more algebraic way of writing that; here's a version (that doesn't care about ambiguity)

readMaybe :: Read a => String -> Maybe a readMaybe    = join . fmap no_trailing_garbage . listToMaybe . reads      where no_trailing_garbage = fmap fst . check (all isSpace . snd)

check :: (MonadPlus m) => (a -> Bool) -> a -> m a check p a    | p a = return a    | otherwise = mzero

I tried Hoogling for a function like check, but couldn't find it. Surely there's one in a library somewhere? It looks useful to me. (I'm rather taken by way the "check (all isSpace . snd)" part reads)

Monad.guard comes close but fails to get the cigar; in fact

guard b == check (const b) ()

So check is more general.

Also, I don't see a singletonListToMaybe that one could use in place of listToMaybe to require unambiguity. Could do

isSingleton [a] = True isSingleton _ = False

and then use "listToMaybe . join . check isSingleton" -- aha! Another use for check!

 Jón

[Footnote: I thought of writing "guard == flip (check . const) ()" but then realised it was pointless]

-- Jón Fairbairn                                 Jon.Fairbairn@cl.cam.ac.uk

You can use the Kleisli composition operator (<=<) to make it a little nicer. singletonListToMaybe :: [a] -> Maybe a singletonListToMaybe [x] = Just x singletonListToMaybe _ = Nothing check :: MonadPlus m => (a -> Bool) -> a -> m a check p a | p a = return a | otherwise = mzero readMaybe = fmap fst.check (all isSpace.snd) <=< singletonListToMaybe.reads Alex