99 questions/Solutions/26

-- Import the 'tails' function
--   > tails [0,1,2,3]
--   [[0,1,2,3],[1,2,3],[2,3],[3],[]]
import Data.List (tails)
 
-- The implementation first checks if there's no more elements to select,
-- if so, there's only one possible combination, the empty one,
-- otherwise we need to select 'n' elements. Since we don't want to
-- select an element twice, and we want to select elements in order, to
-- avoid combinations which only differ in ordering, we skip some
-- unspecified initial elements with 'tails', and select the next element,
-- also recursively selecting the next 'n-1' element from the rest of the
-- tail, finally consing them together
 
-- Using list comprehensions
combinations :: Int -> [a] -> [[a]]
combinations 0 _  = [ [] ]
combinations n xs = [ y:ys | y:xs' <- tails xs
                           , ys <- combinations (n-1) xs']
 
-- Alternate syntax, using 'do'-notation 
combinations :: Int -> [a] -> [[a]]
combinations 0 _  = return []
combinations n xs = do y:xs' <- tails xs
                       ys <- combinations (n-1) xs'
                       return (y:ys)

combinations :: Int -> [a] -> [[a]]
-- We don't actually need this base case; it's just here to
-- avoid the warning about non-exhaustive pattern matches
combinations _ [] = [[]]
-- Base case--choosing 0 elements from any list gives an empty list
combinations 0 _  = [[]]
-- Get all combinations that start with x, recursively choosing (k-1) from the
-- remaining xs. After exhausting all the possibilities starting with x, if there
-- are at least k elements in the remaining xs, recursively get combinations of k
-- from the remaining xs.
combinations k (x:xs) = x_start ++ others
    where x_start = [ x : rest | rest <- combinations (k-1) xs ]
          others  = if k <= length xs then combinations k xs else []