On Sat, 2007-04-28 at 06:45 -0700, Stefan O'Rear wrote:
On Sat, Apr 28, 2007 at 03:49:44PM +0200, Hans van Thiel wrote:
Hello All,
The standard function groupBy of List.hs doesn't work as I expect in this case:
groupBy (\x y -> (last x) == (last y)) ["abc", "bd","cac"]
results in:
[["abc"],["bd"],["cac"]]
where I want:
[["abc","cac"], ["bd"]]
Am I doing something wrong, or is this a bug? The function is defined (in List.hs in the hugs Data library) as :
-- | The 'groupBy' function is the non-overloaded version of 'group'. groupBy :: (a -> a -> Bool) -> [a] -> [[a]] groupBy _ [] = [] groupBy eq (x:xs) = (x:ys) : groupBy eq zs where (ys,zs) = span (eq x) xs
You are doing something wrong. groupBy is specified to never reorder elements. You probably want to use sortBy first.
Stefan Ah, many thanks...
Hans