On Friday 04 March 2011 01:18:07, Jacek Generowicz wrote:
Hi Cafe,
It seems that I don't understand what groupBy does.
I expect it to group together elements as long as adjacent ones satisfy the predicate, so I would expect ALL four of the following to give one group of 3 and a group of 1.
Prelude> :m + Data.List Prelude Data.List> groupBy (<) "abcb" ["abcb"] Prelude Data.List> groupBy (<) "abca" ["abc","a"] Prelude Data.List> groupBy (<) [1,2,3,2] [[1,2,3,2]] Prelude Data.List> groupBy (<) [1,2,3,1] [[1,2,3],[1]]
What am I missing?
That groupBy expects an equivalence relation (iirc, that was documented some time, seems to be gone, there's only a hint left at the docs for group "equality test"). It tests subsequent elements against the first of the group, not adjacent elements.