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

Jon Fairbairn jon.fairbairn at cl.cam.ac.uk
Thu Jul 2 06:36:09 EDT 2009

Dan Doel <dan.doel at gmail.com> 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
    = 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!


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

Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk

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