encode.hs

import Control.Monad

import Data.List

dict = ['a' .. 'z']

base = length dict + 2

encode :: String -> Int
encode = fst . foldl go (0, 0)
    where
        go (acc, p) v =
            let Just i = findIndex (v==) dict
            in (acc + (i + 1) * base ^ p, p + 1)

decode :: Int -> String
decode = unfoldr go
    where
        go r = do
            let (r', s) = r `divMod` base
            guard (s /= 0)
            Just (dict !! (s - 1), r')