Dear Cafe - what is the ordering of the output of https://hackage.haskell.org/package/base-4.16.0.0/docs/src/Data.OldList.html... - and why is it not lexicographic? E.g., *Main> last $ L.permutations [1..8] [5,3,6,2,7,1,8,4] note: [5,_,6,_,7,_,8,_] and [_,3,_,2,_,1,_,4] indeed, an auxiliary function is called "interleave". Data.List.permutations appeared in base-4.0.0.0 (ghc-6.10.1, 2008) https://downloads.haskell.org/~ghc/6.10.1/docs/html/users_guide/release-6-10... What is the source of the algorithm? Is it any of the algorithms in Knuth AOCP 4A(I) 7.2.1.2? Hard to tell - as they are given in an imperative style. The implementation *is* fast, e.g., L.sort . L.permutations seems better than a naive lexicographic enumeration lp1 [] = [[]] lp1 xs = do (ys, z:zs) <- zip (L.inits xs) (L.tails xs) (z:) <$> lp1 (ys <> zs) (un-scientifically measured with :set +s) Here, clearly, L.inits and "<>" are bad. NB - I was looking in to this because of https://www.mathekalender.de/wp/calendar/challenges/challenge-01/ Yes I know that can be solved without enumeration. - J.