On 05/01/2012 11:09, Brandon Allbery wrote:

On Thu, Jan 5, 2012 at 05:57, Steve Horne <sh006d3592@blueyonder.co.uk> wrote:

-- groupCut - Similar to groupBy, but where groupBy assumes an equivalence relation,
-- groupCut takes a function that indicates where to cut. The two parameters to this
-- function are always adjacent items from the list, and if the function returns True,
-- a cut is done between the two items.

span/break?

Using those, the test function won't always be passed two *adjacent* elements from the list. After all, they're based on takeWhile and dropWhile, which take unary functions, meaning an element has already been curried in (the starting element of the group).

That's probably how the current groupBy is implemented - the approach that assumes an equivalence relation, giving unexpected results when the By function isn't an equivalence relation.