Ways to pack (adjacent) elements of a list into 2-tuples

Question

I was wondering if there would be a concise/one-liner way to do the following:

pack :: [a] -> [(a, a)]
pack []       = []
pack [_]      = []
pack (x:y:xs) = (         


        

Answer 1

Note that for xs = [x1, x2, ..., xn-1, xn], we have

init xs = [x1, x2, ... , xn-1]
tail xs = [x2, x3, ... , xn  ]

leading to

zip (init xs) (tail xs) = [(x1, x2), (x2, x3), (x3, x4), ...]

and what we want is

pack xs                 = [(x1, x2),           (x3, x4), ...]

which is easy to get once we have a list of masks

cycle [True, False]     = [ True,    False,    True, ...    ]

leading to the one-liner

pack :: [a] -> [(a, a)]
pack xs = map snd . filter fst . zip (cycle [True, False]) $ zip (init xs) (tail xs)