[Haskell-cafe] Behavior of groupBy

[Stefan O'Rear]

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