On Fri, May 8, 2009 at 11:22 AM, aditya siram
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
There are a number of ways of doing this, none of them entirely satisfactory. The essential problem is that subtraction becomes a partial function; i.e. if you try to subtract a and b, there's no way for the compiler to statically know if that's going to be nonnegative or not. One option would be to make a newtype of Integer, define (+), (*), etc. as with regular integers, define abs = id, negate = undefined. Then you can either make (-) a partial function (as in (NonNeg x) - (NonNeg y) = case x - y of z | z >= 0 -> z | otherwise -> undefined ) or leave subtraction undefined and make a maybeSubtract function that returns a Maybe value based on whether or not the result would be positive or negative. A third option would be to just take the absolute value of the result of a subtraction, although this should be used ONLY in situations where it is appropriate because usually you want to distinguish between x and -x. You could also use Peano numerals, but I don't think that those are very efficient. That's where you define data Nat = Zero | Succ Nat and so 3 :: Nat = Succ (Succ (Succ Zero)) etc. Hope that's of some help to you. Alex