Re: [Haskell-cafe] Please help with double recursion

On Saturday 28 May 2011 14:19:18, Dmitri O.Kondratiev wrote:

Thanks for simple and beautiful code to get all pairs. Yet, I need to get to the next step - from all pairs to build all chains, to get as a result a list of lists:

[[abcde, acde, ade, ae,] [bcde, bde, be,] [cde, cd, ce,] de]]

This is where I got stuck.

-- instead of pairing with a single later element, we cons with the -- tail beginning at that later element chainsFromFirst :: [a] -> [[a]] chainsFromFirst (x:xs) = [x:ys | ys <- init (tails xs)] chainsFromFirst [] = [] -- we need init (tails xs) becuase we don't want the final empty tail allChains :: [a] -> [[a]] allChains xs = tails xs >>= chainsFromFirst -- we could use init (tails xs) here too, but that is not necessary