Re: [Haskell-cafe] golf, predicate check function for MonadPlus (was Re: How to read safely?)
On Thursday 02 July 2009 6:36:09 am Jon Fairbairn wrote: 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. I've often noticed the need for a similar function in conjunction with unfoldr: -- This is overly general for unfoldr, but it lines up with check stopAt :: (MonadPlus m) = (a - Bool) - (a - b) - a - m b stopAt p f x | p x = mzero | otherwise = return (f x) -- stopAt p f x = guard (not $ p x) return (f x) -- stopAt p f = liftM2 () (guard . not . p) (return . f) -- etc. Then you can write: unfoldr (stopAt p $ f) where p is a stopping predicate based on the seed, and f unfolds the seed one step. This lets you use the many functions in the standard library that have types like: s - (a, s) where unfoldr wants them to instead be: s - Maybe (a, s) However, I don't really like the name stopAt, and have never come up with anything better. And of course: check = flip stopAt id . not -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] golf, predicate check function for MonadPlus (was Re: How to read safely?)
On Mon, Jul 6, 2009 at 8:49 PM, Dan Doeldan.d...@gmail.com wrote: I've often noticed the need for a similar function in conjunction with unfoldr: -- This is overly general for unfoldr, but it lines up with check stopAt :: (MonadPlus m) = (a - Bool) - (a - b) - a - m b stopAt p f x | p x = mzero | otherwise = return (f x) -- stopAt p f x = guard (not $ p x) return (f x) -- stopAt p f = liftM2 () (guard . not . p) (return . f) -- etc. Then you can write: unfoldr (stopAt p $ f) I have the following function sitting around: unfoldUntil :: (b - Bool) - (b - (a, b)) - b - [a] unfoldUntil p f n = unfoldr g n where g m | p m = Nothing | otherwise = Just $ f m But I don't remeber where I picked it up from. It looks like it fills a similar niche. Antoine ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] golf, predicate check function for MonadPlus (was Re: How to read safely?)
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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 Fairbairnjon.fairba...@cl.cam.ac.uk wrote: 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 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
