Have you tried BangPatterns? Compiled with optimization, I get 22 secs. Here's the full program:

{-# LANGUAGE BangPatterns #-}

f :: Int -> Int -> Int
f !m !n
   | m==0      = n+1
   | n==0      = f (m-1) 1
   | otherwise = f (m-1) (f m (n-1))

main = putStrLn (show (f 4 1))

-- Kim-Ee

On Fri, Dec 11, 2015 at 9:47 PM, Abhishek Kumar <abhishekkmr18@gmail.com> wrote:

I was trying to write below program for ackerman function but it fails (waits too long) for ack(4,1) whereas a recursive C program gives result in 37secs.Can someone pls explain this behaviour and recomend some optimisation.

------haskell code
f m n | m==0 =n+1
| n==0 = f (m-1) 1
| otherwise = f (m-1) (f m (n-1))

Thanks
Abhishek Kumar

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