Integers to intervals (Haskell solution)

This is the known task ("problem" from Clojure problems list): to convert integers list to intervals by merging they... See:

[1,2,3,5,7,6,10] -> [<1,3>,<5,7>,10]

First, I create type El which is input list element and can be point or already interval (with simple factory iv). Relation Rel is needed to determine relative location of point and interval. Join of point to interval is based on this result. joinToIv joins point to interval and returns possible new one. Function joinToIvs joins point to one of intervals but if it is not possible then returns original intervals with new one added. Main algorithm is a left folding of joinToIvs.

Code is very dirty and sure may be more short :-)

-- Dirty code of turn integers to intervals
module Intervals where

-- element is interval Iv of 2 integers or point of 1 integer
data El = Iv Integer Integer|Pt Integer
instance Show El where
  show (Iv n m)|n == m = show n
               |otherwise = "<" ++ show n ++ "," ++ show m ++ ">"
  show (Pt n) = show n

-- constructor for intervals from 2 integers
iv :: Integer -> Integer -> El
iv n m|n < m     = Iv n m
      |otherwise = Iv m n

-- relation pt/interval: L (left), LS (left-side), IN (included), RS (right-side), R (right)
data Rel = L|LS|IN|RS|R deriving Show

-- relation
rel :: El -> El -> Rel
rel (Iv a b) (Pt n)  |a - n == 1  = LS
                     |a - n > 1   = L
                     |b - n >= 0  = IN
                     |b - n == -1 = RS
                     |otherwise   = R
rel (Iv a b) (Iv c d)|c - b == 1 = RS
                     |otherwise  = R

-- join integer to element (which is expected to be interval)
-- joinToIv :: El -> Integer -> [El]
joinToIv :: El -> Integer -> Maybe El
joinToIv e@(Iv a b) n = let p = Pt n in
  case rel e p of
    LS -> Just $ iv n b
    IN -> Just e
    RS -> Just $ iv a n
    otherwise -> Nothing

-- join integer to list of intervals by extend some or add new one to the end
joinToIvs :: [El] -> Integer -> [El]
joinToIvs [] n = [iv n n]
joinToIvs (e:es) n =
  maybe ([e] ++ joinToIvs es n) (\e' -> _add e' es) (joinToIv e n) where
  _add (Iv a b) ((Iv c d):es1)|abs (c-b) == 1 = [Iv a d] ++ es1
  _add e1 es1 = [e1] ++ es1

-- Turn integers to intervals
toIv :: [Integer] -> [El]
toIv [] = []
toIv (n:ns) =
  foldl (joinToIvs) [iv n n] ns

--------------------------------------------------
main :: IO ()
main = print l >> print (toIv l) where
  l = [1,2,3,5,6,4,0,10,11,9,12,100,-1,80,99,101,98,8,7]