Dave Feustel:
Talk about synchronicity! I was just wondering whether 'weaving' of infinite lists is possible.
eg weave the infinite lists [2,4..], [3,6..], [5,10..] to get [2,3,4,5,6,8,9,10,..]
Is this kind of lazy evaluation possible?
The base library version of (concat . transpose) can do that, since for infinite lists, you don't have the termination requirements of the OP. By the way, there is an error in my previous version of weave: *Main> weave [[1,1,1,1],[2,2],[3,3,3]] [1,2,3,1,2,3,1,1] Dan's version also has this behaviour. So, a correct list-based solution that doesn't use reverse or quadratic concatenation isn't immediately obvious. However, Chris Mears' solution can easily be adapted to use the O(1) snoc from Data.Sequence:
import Data.Sequence
weave = weaveSeq . fromList where weaveSeq xs = case viewl xs of (x:xs) :< xss -> x : weaveSeq (xss |> xs) _ -> []