Hello Duncan, Friday, November 3, 2006, 4:42:27 PM, you wrote:
groupBy ((==) `on` fst) vs groupBy (equating fst)
groupOn fst sortOn snd -- |Sort list by function result (use Schwarznegian transform) sortOn f = map snd . sortOn' fst . map (keyval f) -- |Sort list by function result (don't use Schwarznegian transform!) sortOn' f = sortBy (map2cmp f) -- |Group list by function result groupOn f = groupBy (map2eq f) -- |Sort and Group list by function result sort_and_groupOn f = groupOn f . sortOn f sort_and_groupOn' f = groupOn f . sortOn' f -- |Check that all adjacent values in the list meets given criteria isAll f [] = True isAll f [x] = True isAll f (x:y:ys) = f x y && isAll f (y:ys) -- |Check that list is sorted by given field/critery isSortedOn f = isAll (<=) . map f -- |Check that all elements in list are equal by given field/critery isEqOn f = isAll (==) . map f -- Utility functions for list operations keyval f x = (f x, x) -- |Return pair containing computed key and original value map2cmp f x y = (f x) `compare` (f y) -- |Converts "key_func" to "compare_func" map2eq f x y = (f x) == (f y) -- |Converts "key_func" to "eq_func" while we are here, i also propose to export from Data.List 'merge' function that is useful on its own -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com