Dan Doel <dan.d...@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
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.fairba...@cl.cam.ac.uk
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