encode tuples into numbers

gistfile1.hs

import Data.List
import Maybe

-- taken from
-- http://www.haskell.org/haskellwiki/Prime_numbers_miscellaneous#Prime_Wheels
primes = 2:3:primes'
         where
           l:p:candidates = [6*k+r | k <- [0..], r <- [1,5]]
           primes' = p : filter isPrime candidates
           isPrime n = all (not . divides n)
                       $ takeWhile (\p -> p*p <= n) primes'
           divides n p = n `mod` p == 0

isPrime n = all not [n `mod` p == 0 | p <- takeWhile (\p -> p*p <= n) primes]

ascendlist [] = []
ascendlist (h:t) = h : (ascendlist (map (+ h) t))

deascendlist [] = []
deascendlist (h:t) = h : map (flip (-) h) (deascendlist t)

encode l = encode' (ascendlist l) where
  encode' [] = 1
  encode' (h:t) = (primes !! (fromIntegral h)) * (encode' t)

getsprime n = if isPrime n then n else fromJust (find (\x -> n `mod` x == 0) primes)

decode n = deascendlist (decode' n) where
  decode' 1 = []
  decode' n = (fromIntegral pi) : (decode' (n `div` p)) where
    p = getsprime n
    pi = fromJust (findIndex (== p) primes)