Sorry for the terrible subject line, but I'm a but stuck here. A couple days ago I overhauled Data.Sequence.splitAt, greatly improving its performance. You can see the new implementation at https://github.com/haskell/containers/blob/e8b1f664a631e3795dfd14f2d8c2b39c9... . There's one spot where I'm still stuck, however. Much like the original implementation, I check that the splitting index is before the end of the sequence (to ensure correctness). I go further, in fact, checking that the splitting index is positive, in order to avoid allocating a new tree-top in a trivial split. Logically, this check should be moved into the Deep case branch in splitTreeE, to avoid pattern matching on the top of the tree twice and performing redundant comparisons. However, when I make that move, the split/append benchmark gets worse. When I looked at the Core, it seemed that I confused GHC somehow. I ended up with a join point taking an extra argument that it totally ignored. I can fix that benchmark by marking the splitTreeMiddleE function INLINE, but then the zipWith benchmark went screwy. Unless I can get non-fragile results, I'm just going to accept the redundant case match. If anyone has any ideas, please let me know. Thanks, David Feuer