Hello,

I'm learning Haskell, so I was attempting memoization based upon the Fibonacci examples but for the Ackermann function. In my tests, I found what seems to be truncated output. See my comments at the end of the code for the test cases and output.

### Begin Code ###

module Main where

import Data.Array


main = do
  let m = 3

      n = 1
      a = ackermann_mem m n

      putStrLn("Ackermann-mem " ++ show m ++ " " ++ show n ++ " = " ++ show a)

 
-- Functions.
-- Based upon examples from:
-- http://reddit.com/r/programming/info/16ofr/comments)

tabulate bounds f = array bounds [(i, f i) | i <- range bounds]

dp bounds f = (memo!) where memo = tabulate bounds (f (memo!))


-- Trying to apply memoization function to Ackermann.
ackermann_mem m n = dp ((0,0), (30, 1000)) ack (m, n)

  where
    ack rec (0, n) = n + 1

    ack rec (m, 0) = rec (m - 1, 1)

    ack rec (m, n) = rec (m - 1, rec (m, n - 1))

    
{-
Test cases:
ackermann_mem 4 1 = 533 -- when using (30, 1000) as upper bound.
ackermann_mem 4 1 = 5533 -- when using (30, 10000) as upper bound. 
ackermann_mem 4 1 = 65533 -- when using (30, 100000) as upper bound. <--- correct answer!

-}

### End Code ###

It seems if I don't choose an upper bound pair for (m,n) that is large enough I get truncated output for the answer, instead of GHC giving me an array index exception... This behavior seems very odd to me, can someone explain? Or is this a bug?

Thank you.
__
Donnie Jones