memoization in Haskell

fib.hs

import qualified Data.Map as M
import Data.Map ((!))

fib :: Integer -> Integer
fib n = fst $ fib_aux M.empty n

fib_aux mp 0 = (1,mp)
fib_aux mp 1 = (1,mp)
fib_aux mp n | M.member n mp = (mp!n,mp)
	     | otherwise = (next,nmp)
	     	where
			(n1,mp1) = fib_aux mp (n-1)
			(n2,mp2) = fib_aux mp1 (n-2)
			next = n1+n2
			nmp = M.insert n next mp2

main = readLn >>= print . fib