LinuxQuestions.org - Haskell: Integral Parameter

Noob Haskell question: Let's say that I have a function that calculates the cost of booking airline tickets according to some market-driven formula:

Code:

-- total cost for n numbers of persons traveling together 

cost 0 = 0

cost 1 = 1000

cost n = if n < 0 then error "cost of negative travelers invalid"

        else if n >= 10 then 700 * n

              else 1000 * n - 500

Now, the only thing that seems lacking here is that I know that in /all/ cases, n is going to be a whole number. But how to I specify that?

After some experimentation I discovered that if I passed in a special option to the compiler I could do this:

Code:

cost (n :: Integer) = if n < 0 then error "cost of negative travelers invalid"

        else if n >= 10 then 700 * n

              else 1000 * n - 500

However, I actually want it to be any whole number type (i.e., Int or Integer) so this is not satisfactory.