On Tue, 23 Jun 2009 14:58:11 +0200, Brandon S. Allbery KF8NH wrote:

On Jun 22, 2009, at 06:03 , Ivan Uemlianin wrote:

...

I'm learning Haskell from a background in Python, and I'm just looking at the sort and sortBy functions in Data.List. In Python, the decorate-sort-undecorate pattern is a popular alternative to using an explicit compare function. For example, to sort a list of lists by

It's fairly common, considering that decorate-sort-undecorate is a functional programming idiom dating back to Lisp. In Haskell it's usually expressed with the decoration in a tuple such that the default sort can be used.

...

map snd . sort . map (\x -> (x,decorate x))

Fancier versions use arrows to make the decorate part cleaner:

...

map snd . sort . map (decorate &&& id)

The simplest form for e.g. sorting by length is:

sortByLength = sortBy (comparing length)

-- Met vriendelijke groet, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

Daniel Fischer wrote:

Prelude> :set +s Prelude> let lens :: [Int]; lens = [(k^2+3*k-2) `mod` 5431 | k <- [1 .. 500]] (0.04 secs, 6184112 bytes) Prelude> let lists = map (flip replicate ()) lens (0.00 secs, 609084 bytes) Prelude> :m +Data.List Prelude Data.List> :m +Data.Ord Prelude Data.List Data.Ord> let srtl1 = sortBy (comparing length) lists (0.00 secs, 0 bytes) Prelude Data.List Data.Ord> let srtl2 = map snd . sortBy (comparing fst) $ map (\l -> (length l, l)) lists (0.02 secs, 5975640 bytes) Prelude Data.List Data.Ord> length (srtl2 !! 420) 4471 (0.19 secs, 37089168 bytes) Prelude Data.List Data.Ord> length (srtl1 !! 420) 4471 (1.09 secs, 542788 bytes)

Profiling too! Excellent. So this shows that with these long lists of lists, the dsu version was about ten times faster. I'll report back once I've written up. Thanks to everyone for your comments. Ivan -- ============================================================ Ivan A. Uemlianin Speech Technology Research and Development ivan@llaisdy.com www.llaisdy.com llaisdy.wordpress.com www.linkedin.com/in/ivanuemlianin "Froh, froh! Wie seine Sonnen, seine Sonnen fliegen" (Schiller, Beethoven) ============================================================

Dear All

I'll report back once I've written up. I hope this doesn't count as blog spam.

I've written up the new Haskell language features from the responses to my query on decorate-sort-undecorate: http://llaisdy.wordpress.com/2009/07/11/decorate-sort-undecorate-in-haskell-... dot (function composition) and dollar look very handy, and of course lambda. The Arrow robot &&& I think I shall leave aside for now.

Thanks to everyone for your comments. And best wishes

Ivan -- ============================================================ Ivan A. Uemlianin Speech Technology Research and Development ivan@llaisdy.com www.llaisdy.com llaisdy.wordpress.com www.linkedin.com/in/ivanuemlianin "Froh, froh! Wie seine Sonnen, seine Sonnen fliegen" (Schiller, Beethoven) ============================================================