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)
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