Comments on: My shame is complete

By: Porges

Porges — Wed, 12 Mar 2008 04:32:48 +0000

I had a similar version which I posted shortly after I saw this... it doesn’t seem to have shown up in the trackbacks for this post, so I’ll link it manually :)

My version.

By: apfelmus

apfelmus — Thu, 28 Feb 2008 03:12:16 +0000

Seeing Maybe a as the a finite map (container) with only one possible key (think of Data.Map () a), the canonical name for maybeWith would be unionWith. This doesn't conceptually fit into Monad or MonadPlus, so I don't know a really short way to get it from there.

However, the cases maybeWith (++) and maybeWith const are special. The latter is the monoid operation on Maybe [a] and the latter is mplus. (It can be debated whether the Monoid instance is wrong in the sense that we should have mappend = mplus.) In other words, we have

maybeWith (++) = mappend maybeWith const = mplus

And to avoid the potentially "unsafe" call to fromJust (could raise an exception), we can appeal to

x mplus Just y = fromMaybe y x

Putting everything together, we get the solution

...
fizzbuzz = zipWith mappend fizz buzz
numbers = map show [1..]
results = zipWith fromMaybe numbers fizzbuzz

By: liz Hare

liz Hare — Wed, 27 Feb 2008 23:27:41 +0000

I wish I understood these replies, but I can tell tell it’s exciting stuff

By: Dougal

Dougal — Wed, 27 Feb 2008 12:36:39 +0000

Yeah Rob, it’s a wee bit embarrassing considering the less-than-intellectually-heavyweight subject of this post. ;-)

By: Robert Hulme

Robert Hulme — Wed, 27 Feb 2008 12:08:41 +0000

You made programming.reddit!

By: Dougal

Dougal — Wed, 27 Feb 2008 10:22:59 +0000

@Greg:

Yep, you're right. That works perfectly. (I fixed the formatting for you. The comments use Markdown syntax for future reference.)

@Chris:

That looks pretty interesting. I still don't have a proper understanding of continuation use but I can see the gist of your solution.

By: Chris K

Chris K — Wed, 27 Feb 2008 03:52:19 +0000

Well, Robert, you could look at the “main1” version at http://hpaste.org/5934 to see “ContT IO” used. And using a new Monoid in main2 and main3 is one of my favorite styles. It combines the separate Fizz and Buzz specifications to solve the problem. So adding new rules that combine in the same way is trivial. And they all should scale well to large numbers.

By: Greg Buchholz

Greg Buchholz — Wed, 27 Feb 2008 02:04:40 +0000

…there’s supposed to be backquotes around the “mplus”. Might be nice to have a preview feature.

By: Greg Buchholz

Greg Buchholz — Wed, 27 Feb 2008 02:02:39 +0000

What about (untested):

maybeWith f a b = liftM2 f a b `mplus` a `mplus` b

By: Robert Hulme

Robert Hulme — Wed, 27 Feb 2008 02:01:38 +0000

What the hell? There's no way I'm accepting this.

I want a solution with Monad Transformers and Arrows.

What are they teaching you kids these days?