Comments on: Finally some hacking of my own: a counter datatype http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/ Haskell programming journal Fri, 17 May 2013 21:48:47 +0000 hourly 1 http://wordpress.com/ By: dr. k http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-394 Tue, 13 Feb 2007 16:03:01 +0000 http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-394 That’s because Show is doing on-the-fly most of the work that in a Map implementation is done upon insertion. This is a conscious design decision, to have cheap inc.

]]> By: kalmar http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-389 Tue, 13 Feb 2007 03:34:05 +0000 http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-389 No, I mean in response to Mikael’s group . sort thingy. A Show instance shouldn’t, in my opinion, introduce any new constraints other than Show. I’m just saying that if your structure doesn’t require Ord, the Show instance definitely shouldn’t add it. If you change it to use a Map, then you have an Ord constraint, but it’s introduced by the structure not by the Show instance.

]]> By: dr. k http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-388 Tue, 13 Feb 2007 02:41:06 +0000 http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-388 It’s an Eq context. And how else do you propose having a counter over an Eqless map? Data.Map probably requires it too.

]]> By: kalmar http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-387 Tue, 13 Feb 2007 00:52:56 +0000 http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-387 It seems “bad” to have the Show instance require an Ord context.

]]> By: dr. k http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-386 Mon, 12 Feb 2007 10:08:25 +0000 http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-386 Points taken!

]]> By: Mikael Johansson http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-385 Mon, 12 Feb 2007 10:00:05 +0000 http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-385 Why use that double list traversal for dropping an element, instead of using
dec x (C y) = C . deleteFirstsBy (==) y [x]

Your n-inc can also be written using the design choices you’ve made:
ninc n x (C y) = C (y ++ replicate n x)
I would even go so far as to do
toCounter xs = C (concatMap (uncurry replicate) xs)

As for show-ing the whole thing, why not just do
show = unlines . map (\(x,y) -> show x ++ “\t” ++ show y) . (map(\x->(head x,length x))) . group . sort

]]> By: dr. k http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-383 Mon, 12 Feb 2007 00:19:12 +0000 http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-383 Well, as I said, I explicitly wanted very cheap inc. count will be only occasional, and dec/toCounter/fromCounter are included only for completeness’ sake.

]]> By: Twan van Laarhoven http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-382 Sun, 11 Feb 2007 23:58:46 +0000 http://syntaxfree.wordpress.com/2007/02/11/finally-some-hacking-of-my-own-a-counter-datatype/#comment-382 ‘dec’ and ‘count’ will (in the worst case) have traverse the entire list. This means they both are O(n). This makes fromCounter O(n^2)! Using a “Map a Int” is a much better choice, fromCounter would be O(n), and all other operations O(log n).

I am also not sure if your semantics of dec are the best choice. If you simply allow a negative count then the following laws hold:
* inc a . dec a == dec a . inc a == id
* totalCount . dec a == subtract 1 . totalCount
where totalCount = sum . map snd . fromCounter

The implementation also becomes simpler if you use Data.Map, because it handles most stuff for you:
> data Counter a = C { unC :: M.Map a Int }
> empty = C M.empty
> inc a = C . M.insertWith (+) a (+1) . unC
> dec a = C . M.alter dec’ a . unC
where
dec’ Nothing = Just (-1)
dec’ (Just 1) = Nothing
dec’ (Just c) = Just (c – 1)
> count a = M.findWithDefault 0 a . unC
> toCounter = C . M.fromList
> fromCounter = M.toList . unC
> showCounter = unlines . map (\a c -> show a ++ “\t” ++ show c) . fromCounter

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