Mark Jones gave several talks at Advanced Functional Programming 2008. In one of them, he presented approaches to enumerating the elements of various datatypes. I found several interesting things in it, but one function stuck out as being perhaps useful in general. infixr 5 ||| (|||) :: [a] -> [a] -> [a] [] ||| ys = ys (x:xs) ||| ys = x : ys ||| xs It interleaves the elements of two lists. It's defined exactly as (++) with the exception that the arguments are swapped for the recursive application. This works nicely when one wants to merge infinite lists. For example, suppose you want a list of the enumerable numbers with a balance of positive and negative: enums :: (Num a, Enum a) => [a] enums = [0..] ||| map negate [1..] You can't use (++) here, because the left side never completes. Does (|||) seem useful to others? Is it already available in some other form (or in a library) of which I'm not aware? If yes and no are the answers, then I wonder if it's useful enough for Data.List (modulo any expected renaming). Sean