Instance of Monad

Hi all,
I hope here is the correct place where i can find the answer that i'm looking for.
I'm writing a code for the xxtea, it is a modification for the Tiny encryption algorithm. The problem is that when i am running my code it is giving me this:
"Instance of Monad (Array Word32) required for definition of coding"
the code is below; hope some one will reply and tell what is the problem.thnx


type TEA_arr = Array Word32 Word32

coding :: TEA_arr -> Word32 -> Word32 -> [Word32] -> Word32 -> Word32 -> TEA_arr
coding v s e k 0 _ = v
coding v s e k q p = do
let sum = getSum s
e = shiftSum sum
y = v ! (p+1)
z = v ! p
mx = getMx y z sum k e p
newarr = v // [(p,(v ! p)+ mx)]
coding newarr sum e k (q - 1) (p+1)

Instance of gonad more like.