[Haskell-beginners] A Quantity Type - Integer without the Negative #'s

Hi all, Is there a datatype in Haskell that can be used to represent only quantities

= 0? I got bitten by a bug because I forgot to reject an amount that was below zero after applying a decrementing operator. A simple unit test would have caught this, but I was wondering if there was some way of getting the type system to ensure this.

thanks ... -deech