gistfile1.hs

import Data.List

indices [] ix2 = ix2
indices ix1 [] = ix1
indices ((l1,r1):ix1) ((l2,r2):ix2)
  | r1 < r2  = (l1,r1) : indices ix1 ((r1+1,r2):ix2)
  | r1 == r2 = (l1,r1) : indices ix1 ix2
  | r1 > r2  = (l2,r2) : indices ((r2+1,r1):ix1) ix2


length2indices :: [Int] -> [(Int,Int)]
length2indices lns = zip beginnings ends
  where
    edges      = snd $ mapAccumL (\acc l -> let z = (l+acc) in (z,z)) 0 lns
    beginnings = 0 : init edges
    ends       = map (subtract 1) edges

test = foldr1 indices $ map length2indices [[2,3,1,8],[2,2,7],[2,8]]