naive greedy algorithm in Haskell

greedy.hs

import qualified Data.Map as M                                                                                                 
                                                                                                                               
amount = 18                                                                                                                    
                                                                                                                               
coins = [5, 4, 2, 1]                                                                                                           
                                                                                                                               
greedy a cs = fn a cs M.empty                                                                                                  
  where                                                                                                                        
    fn _ [] m = m                                                                                                              
    fn x (y:ys) m = let t = x `div` y                                                                                          
                     in if t > 0 then fn (x-t*y) ys (M.insert y t m)                                                           
                                 else fn x ys m                                                                                
                                                                                                                               
main :: IO ()                                                                                                                  
main = do                                                                                                                      
    let result = greedy amount coins == M.fromList [(1,1),(2,1),(5,3)]                                                         
    putStrLn ("greedy amount coins == M.fromList [(1,1),(2,1),(5,3)] is " ++ show result)