On 5/28/11 8:31 AM, Daniel Fischer wrote:
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
The variant I came up with was: allChains xs = do y:ys <- tails xs zs@(_:_) <- tails ys return (y:zs) Which is essentially the same. I only very rarely use list comprehensions, but if you prefer that syntax you can always do: allChains xs = [ (y:zs) | y:ys <- tails xs, zs@(_:_) <- tails ys ] -- Live well, ~wren