Comments on: Aha-one-liners

By: Ben

Ben — Sun, 25 Nov 2007 22:09:07 +0000

liftM dec . mapM (flip M.lookup decodeMap)

By: Magnus

Magnus — Wed, 24 Oct 2007 08:20:38 +0000

mnislaih, I took the liberty of deleting the “failed” comments. I know that markdown isn’t the most commonly used markup, but it deserves to be. The preview functionality is your friend. On to the contents of your comment. Using fmap or <$> would indeed make it even shorter and sweeter. I suspect I’ll have to play a bit more with Haskell, and read up on what Control.Applicative has to offer.

Ricardo, I suspect there are two back-ticks missing, right?

By: Greg M

Greg M — Wed, 24 Oct 2007 04:07:29 +0000

That is a truly beautiful thing.

By: Kefer

Kefer — Tue, 23 Oct 2007 19:01:05 +0000

re: I can write my function as if failure doesn’t exist.

Right! The title of the paper says it all: "How to convert failure into a list of successes."

Maybe is like the list monad except that (:) on anything other than [] is a no-op.

By: Evan M

Evan M — Tue, 23 Oct 2007 18:33:01 +0000

You could also use mapM in place of your combination of sequence and map. I think of it as "map, sequenced within the monad". If you think about how IO would use it ("do each of these operations in sequence, collecting the results") it makes the translation into the Maybe monad more obvious.

I'd maybe write your code as as: decode = liftM dec . mapM (flip M.lookup decodeMap)

By: Bryan Burgers

Bryan Burgers — Tue, 23 Oct 2007 17:14:58 +0000

Actually, that ’sequence $ map (…) …’ bit is used quite frequently for mapping, when the function of map results in a monadic value. The prelude defines ‘mapM’ for this situation.

By: mnislaih

mnislaih — Tue, 23 Oct 2007 13:56:09 +0000

Sorry, but it is not trivial at all to get the formatting right.

mapM f = sequence . map f

liftM can be replaced by the more abstract fmap, or even better: the applicative combinator <$>.

And we can strip flip by using a section:

dec <$> mapM (`M.lookup` decodeMap) s

By: Ricardo Herrmann

Ricardo Herrmann — Tue, 23 Oct 2007 13:28:12 +0000

A bit of syntax sugar: (flip M.lookup decodeMap) -> (M.lookup decodeMap)

By: she

she — Tue, 23 Oct 2007 08:14:11 +0000

Easy? It does not look easy at all to me