#!/usr/local/bin/runhaskell
import Data.List (sort)

a = [45,30,28,27,20,1,2,3,4,29]
sa = sort a

span :: (Num a, Ord a) => [a]-> String
span [] = ""
span xs = find_span (sx,sx) sxs ""
  where
    (sx:sxs) = sort xs -- ensure list is sorted

    show_pair (low,high) | low == high = show low
    show_pair (low,high) | otherwise   = (show low) ++ "-" ++ (show high)

    find_span state             []     acc               = (acc ++ show_pair state)
    find_span state@(low, high) (x:xs) acc | x == high+1 = find_span (low,x) xs acc
    find_span state@(low, high) (x:xs) acc | x >  high   = find_span (x,x) xs (acc ++ (show_pair state) ++ ",")