Interleave two lists
leather at cs.uu.nl
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
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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