How to coerce a type in SML (like casting)

I'm creating a structure of Rationals (int * int) and one of my functions is:

fun diff ((n, d), (n', d')) = 
  let 
    val (top, bot) = sum ((n, d), (~n', d'))
  in 
    (top / gcd(top, bot), bot / gcd(top, bot))
  end

gcd gives me the greatest common denominator, so I don't end up with 2/8, but rather 1/4 as it should be. gcd uses mod to find the gcd, so it returns an int. But I can't get the expression with division to be typed as an int. When I tried adding : int * int to the end of the diff declaration, it gives me a type error that the expressions real * real and int * int don't match.

How can I force integer division, or cast the expression to an integer? If both are possible, which one is better?

Solution

Yeah you are using the wrong operator. / is the floating-point division operator. div (as mentioned by D.Shawley) is the integer division operator. div is the right thing to use in this case, because you are dividing two integers and want to get an integer.