Interleave two lists

Sean Leather leather at
Sun Aug 24 16:07:52 EDT 2008

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

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